(that is MY_CHECK_C_COMPILER_FLAG, MY_CHECK_CXX_COMPILER_FLAG,
MY_CHECK_AND_SET_COMPILER_FLAG)
This is to have a clear "internal" name prefix and to avoid
mixing them with hand-named symbols
Fix all cmake tests (including plugin) to use
MY_CHECK_AND_SET_COMPILER_FLAG. And fix that function
to be compatible with cmake 3.0. This way flag checks
are correctly cached (even in cmake 3.0) and properly reused.
If a flag is supported only for C or C++ - add it to the
corresponding compiler option list. Old behavior was to
add always to both, but only if supported in both.