This creates variable CMAKE_<LANG>_IMPLICIT_INCLUDE_DIRECTORIES to
specify implicit include directories on a per-language basis. This
replaces the previous platform-wide variable. It is necessary to
avoid explicit specification of -I/usr/include on some compilers
(such as HP aCC) because:
1.) It may break ordering among system include directories defined
internally by the compiler, thus getting wrong system headers.
2.) It tells the compiler to treat the system include directory
as a user include directory, enabling warnings in the headers.
See issue #8598.
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