Fixes: #17997

This family enable to manage link flags

Three new properties:
* directory property: LINK_OPTIONS
* target properties: LINK_OPTIONS and INTERFACE_LINK_OPTIONS

Two new commands
* add_link_options(): to populate directory property
* target_link_options(): to populate target properties

Fixes: #16543

Run the `clang-format.bash` script to update all our C and C++ code to a
new style defined by `.clang-format`.  Use `clang-format` version 6.0.

* If you reached this commit for a line in `git blame`, re-run the blame
  operation starting at the parent of this commit to see older history
  for the content.

* See the parent commit for instructions to rebase a change across this
  style transition commit.

Fixes: #16365

Options specified via `COMPILE_OPTIONS` and `INTERFACE_COMPILE_OPTIONS`
are deduplicated, but individual options can legitimately be duplicated
when grouped with other options, e.g.

    -D A -D B

After deduplication that becomes `-D A B`.  Therefore we need a way to
treat groups of options as units during deduplication.  A simple approach
is to specify each group as one option, e.g.

    "-D A" "-D B"

However, that conflicts with options that legitimately have spaces.  To
break this ambiguity, add a `SHELL:` prefix syntax to specify that an
option should be parsed like shell command line arguments after
deduplication, e.g.

    "SHELL:-D A" "SHELL:-D B"

These will survive deduplication intact, and then be parsed to produce
`-D A -D B` on the final command line.

Fixes: #15826

When evaluating `INTERFACE_SYSTEM_INCLUDE_DIRECTORIES`, or evaluating
`INTERFACE_INCLUDE_DIRECTORIES` on an imported target, thread the
compile language through to the generator expression evaluator so
that it can support `$<COMPILE_LANGUAGE:...>`.

Fixes: #17811

Use `CMAKE_IMPORT_LIBRARY_SUFFIX` to identify platforms that have DLL
import libraries rather than memorizing a list of platform names.

Fixes: #16801

Note: This finally links the object-files of the `OBJECT` library from
the right-hand side of `target_link_libraries` to the target on the
left-hand side. However, this will only happen with directly linked
`OBJECT` libraries, not with `OBJECT` libraries "linked" through
property `INTERFACE_LINK_LIBRARIES` of a target on the right-hand side!

Fixes: #14778

Note: This still does not link the object-files of the `OBJECT` library
from the right-hand side of `target_link_libraries` to the target on the
left-hand side. (In this particular case of another `OBJECT` library on
the left-hand side this would not make any sense anyway. The target on
the left-hand side has no link-step.)

Issue: #14778

Note: This only allows `OBJECT` libraries to be on the right-hand side
of `target_link_libraries` but still does not link its object-files to
the target on the left-hand side.

Issue: #14778

Our `cmake -E __create_def` helper used for `WINDOWS_EXPORT_ALL_SYMBOLS`
and merging of multiple `.def` files is available only with CMake hosted
on Windows.  However, we may generate use of it on other platforms since
commit v3.9.0-rc1~405^2 (Support WINDOWS_EXPORT_ALL_SYMBOLS with `.def`
files, 2017-03-13) when multiple `.def` files are listed.  This results
in a build error because the tool doesn't exist.

Fix our logic to avoid using the tool on non-Windows platforms.  Instead
silently ignore all but the first `.def` source as we did before.

Issue: #17773

* Change some functions to take `std::string` instead of
  `const char*` in the following classes: `cmMakeFile`, `cmake`,
  `cmCoreTryCompile`, `cmSystemTools`,  `cmState`, `cmLocalGenerator`
  and a few others.
* Greatly reduce using of `const char*` overloads for
  `cmSystemTools::MakeDirectory` and `cmSystemTools::RelativePath`.
* Remove many redundant `c_str()` conversions throughout the code.

- Use `std::move` while inserting temporary results into vectors.
- Change `push_back` to `emplace_back` where appropriate.

Some are user-facing. Others are source comments.

explicitLibraries can never be nullptr at this point

Signed-off-by: Matthias Maennich <matthias@maennich.net>

Changes done via `clang-tidy` with some manual fine-tuning
for the variable naming and `auto` type deduction
where appropriate.

Refactoring in commit v3.8.0-rc1~445^2~2 (cmTarget: Move sanity checks
and computed property access to callers, 2016-10-13) exposed a typo in
commit v3.8.0-rc1~445^2~3 (cmGeneratorTarget: Implement
cmTargetPropertyComputer interface, 2016-10-13).  Together they broke
the `$<TARGET_PROPERTY:mytgt,SOURCES>` generator expression in the case
that the `SOURCES` target property is populated in part by the
`target_sources` command.  Add the missing `;`-separator.

Fixes: #17243

Adds cmGeneratorTarget::clearSourcesCache() which clears the cache
of precomputed sources lists and object names. The cache gets
recomputed on demand.

Clearing the cache is necessary in case an OBJECT library
gets a source added after a target (ORIGIN) that includes it
computed it's external sources cache.
This may happen in AUTOMOC when ORIGIN is processed before the
OBJECT library which gets a mocs_compilation.cpp file added.

Closes #17152
Closes #17139
Closes #17085
Closes #15967

The code

    add_library(A OBJECT a.c)
    target_sources(A PRIVATE $<TARGET_OBJECTS:A>)

used to crash CMake via infinite recursion while evaluating the
generator expression.  Then the change in commit v3.9.0-rc1~266^2~1
(cmGeneratorTarget: Replace source classifier implementation,
2017-04-07) avoided the infinite recursion because GetKindedSources now
creates a map entry and initializes it once.  If it is called again on
the same target during that initialization, the partially computed
results are returned.  This is still wrong but does not crash.
Detect and diagnose this case instead.

Co-Author: Ben Boeckel <ben.boeckel@kitware.com>
Fixes: #16578

The logic added in commit v3.6.0-rc1~30^2 (Add a variable to specify
language-wide system include directories, 2016-05-24) to use
`CMAKE_<LANG>_STANDARD_INCLUDE_DIRECTORIES` incorrectly filters them by
`CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES`.  Rather than recognizing
this, commit v3.8.0-rc1~60^2 (Android: Pass sysroot include directory
explicitly, 2017-01-20) worked around the problem by incorrectly
removing `/usr/include` from `CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES`
so it worked in `CMAKE_<LANG>_STANDARD_INCLUDE_DIRECTORIES`.

By not filtering out `/usr/include` from user-specified include
directories, we allow the code

    include_directories(${CMAKE_SYSROOT}/usr/include)

to place the include directory too early on the command line.

Fix support for standard include directories to not be filtered by
implicit include directories, and do not remove `/usr/include` from the
list of implicit include directories for Android builds.  Add a test
case to verify that an explicit `/usr/include` is ignored in favor
of the standard directory at the end.

Fixes: #17059

We only define `INTERPROCEDURAL_OPTIMIZATION` behavior for C, CXX, and
Fortran languages.  Do not try to enable support for other languages.
Furthermore, each language builds with a different compiler, so check
for support by CMake and the compiler for each language independently.

Fixes: #16944