[CMake] buglets in Modules/CMakeCInformation.cmake

[SK]

CMakeCInformation.cmake tests the value of _INCLUDED_FILE under the
incorrect assumption that CMAKE_COMPILER_ID is true.
For example, line 46 and line 57 from a very recent version in the git
repo.  If CMAKE_COMPILER_ID is not true, then the value of
_INCLUDED_FILE is a left-over from earlier unrelated tests.