ENH:  Add -D preprocessor defines to the moc commands.
      Needed to fix #7465.

This introduces the unset() command to make it easy to unset CMake
variables, environment variables, and CMake cache variables.  Previously
it was not even possible to unset ENV or CACHE variables (as in
completely remove them).  Changes based on patch from Philip Lowman.
See issue #7507.

ENH: Add the RECURSE_SYMLINKS_OFF flag to the FILE GLOB_RECURSE command. Exposes the recently added kwsys capability that prevents recursing through symlinks to CMake scripts.

Some native build tools, particularly those for cross compiling, may
have a limit on the length of the full path to an object file name that
is lower than the platform otherwise supports.  This change allows the
limit to be set by the project toolchain file through the variable
CMAKE_OBJECT_PATH_MAX.

ENH: Add RecurseThroughSymlinks data member to kwsys::Glob. Allows recursive globs to skip symlinks when necessary. Default to true for backwards compatible behavior. Used from the ctest coverage handler to avoid recursing through the '/Applications' directory on the Mac looking for *.da files... Should fix the hangs reported recently by Mac CMake dashboard submitters.

It is useful to be able to test if a target has been created.  Often
targets are created only inside conditions.  Rather than storing the
result of the condition manually for testing by other parts of the
project, it is much easier for the other parts to just test for the
target's existence.  This will also be useful when find-modules start
reporting results with IMPORTED targets and projects want to test if a
certain target is available.

According to "man select" on Linux it is possible that select() lies
about data being ready on a pipe in some subtle cases.  We deal with
this by switching to non-blocking i/o and checking for EAGAIN.  See
issue #7180.

BUG: fix for 6794 support for LTCG WholeProgramOptimization, which is not available in VS 8 and newer.

We frequently need to wipe out all the CMake test build directories in
order to run tests from scratch.  This change adds a test_clean custom
target to remove all these directories for out-of-source builds.

The LINK_INTERFACE_LIBRARIES target property may not contain the
"debug", "optimized", or "general" keywords.  These keywords are
supported only by the target_link_libraries (and link_libraries) command
and are not a generic library list feature in CMake.  When a user
attempts to add one of these keywords to the property value, we now
produce an error message that refers users to alternative means.

The LINK_INTERFACE_LIBRARIES property does not apply for STATIC
libraries.  The IMPORTED_LINK_INTERFACE_LIBRARIES property does apply
for STATIC libraries.  State both explicitly in the documentation.
Also, clarify that the per-configuration version of these properties
completely overrids the generic version.

The compatibility check to allow linking to modules should test for
CMake 2.2, not the unreleased 2.3.  See issue #7500.  Furthermore, the
message should be more clear about fixing the code instead of setting
CMAKE_BACKWARDS_COMPATIBILITY unless one is just trying to build an
existing project.

In the future some policies may be set to REQUIRED_IF_USED or
REQUIRED_ALWAYS.  This change clarifies the error messages users receive
when violating the requirements.

When creating an IMPORTED target for a library that has been found on
disk, it may not be known whether the library is STATIC or SHARED.
However, the library may still be linked using the file found from disk.
Use of an IMPORTED target is still important to allow per-configuration
files to be specified for the library.

This change creates an UNKNOWN type for IMPORTED library targets.  The
IMPORTED_LOCATION property (and its per-config equivalents) specifies
the location of the library.  CMake makes no assumptions about the
library that cannot be inferred from the file on disk.  This will help
projects and find-modules import targets found on disk or specified by
the user.

In switch statements that deal with only a few target types, use a
'default' case for the remaining target types instead of listing them
explicitly.  This will make it easier to add more types in the future.

Rename the recently added INTERFACE mode of the target_link_libraries()
command to LINK_INTERFACE_LIBRARIES.  This makes it much more distinct
from a normal call to the command, and clearly states its connection to
the property of the same name.  Also require the option to appear
immediately after the target name to make it a mode rather than an
option.

It is likely that projects or CMake modules in the future will need to
check the value of a policy setting.  For example, if we add a policy
that affects the results of FindXYZ.cmake modules, the module code will
need to be able to check the policy.