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Create a cmLinkImplItem class derived from cmLinkItem so more
information can be added to link implementation entries than link
interface entries.  Convert the LinkImplementation Libraries member to
hold it.  Update client sites accordingly.

Instead of storing just the string names in these structures, lookup any
target associated with each item and store its cmTarget pointer.  Use
the cmLinkItem class to hold the name and pointer together.  Update
client sites to use the pre-stored lookup result instead of looking up
the target name again.

Create a cmTarget::LookupLinkItems helper method to handle the lookup.
Since lookups are now moving from cmComputeLinkDepends::AddLinkEntries
to cmTarget::LookupLinkItems, move use of CheckCMP0004 to the latter.
This drops use of CheckCMP0004 from entries added for _LIB_DEPENDS
variables by cmComputeLinkDepends::AddVarLinkEntries, but I do not
think that use was intentional originally anyway.

Add a method like GetUtilities but that provides the target names
already looked up and resolved to cmTarget pointers internally.  Update
call site in cmComputeTargetDepends::AddTargetDepend to use the
already-found target instead of looking it up again.

Many of the 'head' arguments added by commit v2.8.11~289^2~1 (Make
linking APIs aware of 'head' target, 2013-01-04) turned out not to be
needed.  The "link implementation" of a target never needs to be
computed with anything but itself as the 'head' target (except for
CMP0022 OLD behavior because then it is the link interface).

Remove the unused 'head' target paths.  Add "internal" versions of
cmTarget::GetDirectLinkLibraries and GetLinkImplementationLibraries
to support the CMP0022 OLD behavior without otherwise exposing the
'head' target option of these methods.

The AddInterfaceDepends method is never called with a non-true value for
its 'linking' argument.  Drop it.

Disallow the use of config-specific source files with
the Visual Studio and Xcode generators. They don't have
any way to represent the condition currently.

Use the same common-config API in cmQtAutoGenerators. While
it accepts config-specific files, it doesn't have to support
multiple configurations yet.

Loop over the configs in cmTargetTraceDependencies
and cmGlobalGenerator::WriteSummary and consume all source
files.

Loop over the configs in cmComputeTargetDepends and compute the
object library dependencies for each config.

If CMAKE_BUILD_TYPE is set, and user code contains:

 target_link_libraries(myexe prefix_$<$<CONFIG:Debug>:debug>)

then the computation with an empty config was computing a target-level
dependency on a target or library called prefix_, and a dependency
on a target or library called prefix_debug (as expected).

The existing logic skips 'prefix_' because it is not a known target,
and defers to the link-dependencies logic to find the library. The
link-dependencies logic does not incorrectly handle the config as
cmComputeTargetDepends did, and so did not encounter 'prefix_'
during its computation. This likely had no effect on the generated
buildsystem.

Relieve cmGeneratorTarget of that responsibility.

Use the clang RemoveCStrCalls tool to automatically migrate the
code. This was only run on linux, so does not have any positive or
negative effect on other platforms.

Ben Boeckel authored Feb 10, 2014 and Brad King committed Mar 08, 2014

Casts from std::string -> cmStdString were high on the list of things
taking up time. Avoid such implicit casts across function calls by just
using std::string everywhere.

The comment that the symbol name is too long is no longer relevant since
modern debuggers alias the templates anyways and the size is a
non-issue since the underlying methods are generated since it's
inherited.

Policies CMP0027, CMP0038, and CMP0046 have per-target meaning so
record the policy settings on targets as they are created.

Rolf Eike Beer authored Jan 15, 2014 and Brad King committed Jan 16, 2014

Most callers already have a std::string, on which they called c_str() to pass it
into these methods, which internally converted it back to std::string. Pass a
std::string directly to these methods now, avoiding all these conversions.
Those methods that only pass in a const char* will get the conversion to
std::string now only once.

Added new policy CMP0046 which requires dependencies added by
add_dependencies() to actually exist.

Commit b04f3b9a (Create make rules for INTERFACE_LIBRARY
targets., 2013-08-21) extended the makefile generator to create
build targets for INTERFACE_LIBRARY targets. No other generators
were extended with this feature.

This conflicts with the feature of whitelisting of target properties
read from INTERFACE_LIBRARY targets. The INTERFACE_* properties
of the INTERFACE_LIBRARY may legitimately contain TARGET_PROPERTY
generator expressions for reading properties from the 'head target'.
The 'head target' would be the INTERFACE_LIBRARY itself when creating
the build rules for it, which means that non-whitelisted properties
would be read.

As an INTERFACE_LIBRARY has no direct link dependencies, we can
short-circuit in cmGeneratorExpressionEvaluator and
in cmGlobalGenerator::CheckLocalGenerators.

As they do not generate any output directly, any generate- or install-
related code acn also be short-circuited. Many of the local generators
already do this.

Because only INTERFACE related properties make sense on INTERFACE_LIBRARY
targets, avoid setting other properties, for example via defaults.

Stephen Kelly authored Aug 21, 2013 and Brad King committed Oct 21, 2013

The result is that the depends of the target are created.

So,

 add_library(somelib foo.cpp)
 add_library(anotherlib EXCLUDE_FROM_ALL foo.cpp)
 add_library(extra EXCLUDE_FROM_ALL foo.cpp)
 target_link_libraries(anotherlib extra)

 add_library(iface INTERFACE)
 target_link_libraries(iface INTERFACE anotherlib)

Executing 'make iface' will result in the anotherlib and extra targets
being made.

Adding a regular executable to the INTERFACE of an INTERFACE_LIBRARY
will not result in the executable being built with 'make iface' because
of the logic in cmComputeTargetDepends::AddTargetDepend.

So far, this is implemented only for the Makefile generator. Other
generators will follow if this feature is possible for them.

Make INTERFACE_LIBRARY targets part of the all target by default.
Test this by building the all target and making the expected library
EXCLUDE_FROM_ALL.

This was missing from commit 30962029 (Make targets depend on the
link interface of their dependees., 2012-12-26), which caused
only immeditate entries of the link interface to become target
depends.

Stephen Kelly authored Dec 06, 2012 and Brad King committed Jan 08, 2013

Previously we kept direct link dependencies in OriginalLinkLibraries.
The property exposes the information in the CMake language through the
get/set_property commands.  We preserve the OriginalLinkLibraries value
internally to support old APIs like that for CMP0003's OLD behavior, but
the property is now authoritative.  This follows up from commit d5cf644a
(Split link information processing into two steps, 2012-11-01).

This will be used later to populate the link interface properties when
exporting targets, and will later allow use of generator expressions
when linking to libraries with target_link_libraries.

Also make targets depend on the (config-specific) union of dependencies.
CMake now allows linking to dependencies or not depending on the config.
However, generated build systems are not all capable of processing
config-specific dependencies, so the targets depend on the union of
dependencies for all configs.

Stephen Kelly authored Jan 04, 2013 and Brad King committed Jan 08, 2013

The 'head' is the dependent target to be linked with the current target.
It will be used to evaluate generator expressions with proper handling
of mapped configurations and is used as the source target of properties.

This requires that memoization is done with a key of a pair of target
and config, instead of just config, because now the result also depends
on the target.  Removing the memoization entirely is not an option
because it slows cmake down considerably.

Make it a static method instead of an array. It is safer for the
type checking and if we add a new target type we will be warned to add
a case to the switch.

Commit e01cce28 (Allow add_dependencies() on imported targets,
2010-11-19) forgot to check if the dependee exists before using it.

Imported targets do not themselves build, but we can follow dependencies
through them to find real targets.  This allows imported targets to
depend on custom targets that provide the underlying files at build
time.

Each inter-target dependency may be a 'link' or 'util' dependency.

Strong dependencies (created by add_dependencies) must be honored when
linearizing a strongly-connected component of the target dependency
graph.  The initial graph edges have strong/weak labels and can contain
cycles that do not consist exclusively of strong edges.  The final graph
never contains cycles so all edges can be strong.

Utility dependencies are "strong" because they must be enforced to
generate a working build.  Link dependencies are "weak" because they can
be broken in the case of a static library cycle.

This converts the CMake license to a pure 3-clause OSI-approved BSD
License.  We drop the previous license clause requiring modified
versions to be plainly marked.  We also update the CMake copyright to
cover the full development time range.

This global property disallows cycles in the inter-target dependency
graph even among STATIC libraries.  See issue #9444.

When an executable target within the project is named in
target_link_libraries for another target, but the executable does not
have the ENABLE_EXPORTS property set, then the executable cannot really
be linked.  This is probably a case where the user intends to link to a
third-party library that happens to have the same name as an executable
target in the project (or else will get an error at build time).  We
need to avoid making the other target depend on the executable target
incorrectly, since the executable may actually want to link to that
target and this is not a circular depenency.

When reporting the dependencies in a strongly connected component quote
the target names to make the message more readable no matter the target
name.

  - Move Tarjan algorithm from cmComputeTargetDepends
    into its own class cmComputeComponentGraph
  - Use cmComputeComponentGraph to identify the component DAG
    of link dependencies in cmComputeLinkDepends
  - Emit non-trivial component members more than once but always
    in a contiguous group on the link line

  - Cycles may be formed among static libraries
  - Native build system should not have cycles in target deps
  - Create cmComputeTargetDepends to analyze dependencies
  - Identify conneced components and use them to fix deps
  - Diagnose cycles containing non-STATIC targets
  - Add debug mode property GLOBAL_DEPENDS_DEBUG_MODE
  - Use results in cmGlobalGenerator as target direct depends