Clang: Add separate "AppleClang" compiler id

Apple distributes their own Clang build with their own version numbers
that differ from upstream Clang.  Use the __apple_build_version__ symbol
to identify the Apple Clang compiler and report the Apple Build Version
as the fourth version component in CMAKE_<LANG>_COMPILER_VERSION.  Add
Compiler/AppleClang-<lang> and Platform/Darwin-AppleClang-<lang> modules
that simply include the upstream equivalents.

Fix comparisons of CMAKE_<LANG>_COMPILER_ID to Clang in CMake's own
source and tests to account for AppleClang.
Add policy CMP0025 for Apple Clang compiler id compatibility

The parent commit introduced a separate "AppleClang" compiler id for
Apple's Clang distribution.  Add a policy in order to support projects
that expect this compiler's id to be just "Clang".  When the policy is
OLD or not set, map AppleClang back to Clang.  Continue to use the
AppleClang id internally while enabling the language, but set the
CMAKE_<LANG>_COMPILER_ID after project() or enable_language() to the
compatible value for use by project code.
Set policy CMP0025 to NEW while building CMake itself

CMake is aware of the policy's NEW behavior and the AppleClang compiler
id.  Set the policy to NEW explicitly to avoid the warning and get the
NEW behavior.

Also teach the RunCMake test infrastructure to build tests with
-DCMAKE_POLICY_DEFAULT_CMP0025=NEW to avoid the policy warning
in test output that must match specific regular expressions.
Merge topic 'apple-clang-id' into next

1763c31 Set policy CMP0025 to NEW while building CMake itself
aa53ee5 Add policy CMP0025 for Apple Clang compiler id compatibility
ab65862 Clang: Add separate "AppleClang" compiler id
Merge topic 'FindGTK2-quiet'

1bbc766 FindGTK2: Search for modules quietly when needed
d9eb277 FindGTK2: Find freetype quietly
675a4d3 FindGTK2: Mark GTK_*_INCLUDE_DIR as advanced
Merge topic 'INTERFACE_LIBRARY-target-type'

ce0c303 install: Teach EXPORT option to handle INTERFACE_LIBRARY targets
435c912 export: Add support for INTERFACE_LIBRARY targets
fe73226 Add the INTERFACE_LIBRARY target type.
Merge topic 'clang-cl-compiler'

3d8356d Clang: Support Windows variants for GNU and MSVC (#13035, #14458)
51ab85c CMakeDetermineCompilerId: Add notion of "simulated" id/version
be10826 CMakeDetermineCompilerId: Fix local var init
Merge topic 'export-policy'

904ff9f export: Add policy CMP0024 to disallow include() of export files
Merge branch 'master' into next