Intuitively one would use CMAKE_C_FLAGS, CMAKE_CXX_FLAGS and
CMAKE_EXE_LINKER_FLAGS to inject custom flags into the build. However
this clashes with the use of cmake cache files which want to add flags as well.
A user specifying CMAKE_C_FLAGS would (accidentally) override the flags set by the cache files.
The solution here is to introduce a new set of flags called
TEST_SUITE_C_FLAGS, TEST_SUITE_CXX_FLAGS and TEST_SUITE_EXE_LINKER_FLAGS
as a means for users to specify additional flags. This way the flags
will not override the settings from the cache file.
In order to educate users to use the new flags a warning message is
displayed if CMAKE_C_FLAGS is set.