Since commit v3.3.0-rc1~397^2 (Check*CompilerFlag: Refactor method used
to pass flags, 2015-02-24) these check modules pass the flags to the
compiler front-end during linking as well as during compilation. This
breaks checks for flags like '-x c++' that are meant only for the
compilation step. Revert the change and add a test covering a
compiler-only flag.
Refactor the Check*CompilerFlag modules to pass the flags to
Check*SourceCompiles using CMAKE_REQUIRED_FLAGS instead of
CMAKE_REQUIRED_DEFINITIONS. Both end up being used, but the variable
for "FLAGS" is more precise.