CMP0025
-------

Compiler id for Apple Clang is now AppleClang.

CMake >= 2.8.13 recognize that Apple Clang is a different compiler
than upstream Clang and that they have different version numbers.
CMake now prefers to present this to projects by setting
CMAKE_<LANG>_COMPILER_ID to "AppleClang" instead of "Clang".  However,
existing projects may assume the compiler id for Apple Clang is just
"Clang" as it was in CMake < 2.8.13.  Therefore this policy determines
for Apple Clang which compiler id to report in
CMAKE_<LANG>_COMPILER_ID after <LANG> is enabled by the project() or
enable_language() command.

The OLD behavior for this policy is to use compiler id "Clang".  The
NEW behavior for this policy is to use compiler id "AppleClang".

This policy was introduced in CMake version 2.8.13.  CMake version
|release| warns when the policy is not set and uses OLD behavior.  Use
the cmake_policy command to set it to OLD or NEW explicitly.