Most of the time, flags are compiler-specific. Our current example will only work with GCC and Clang; compilers from other vendors will not understand many, if not all, of those flags. Clearly, if a project aims at being truly cross-platform, this problem has to be solved. There are three approaches to this.
The most typical approach will append a list of desired compiler flags to each configuration type CMake variable, that is, to CMAKE_<LANG>_FLAGS_<CONFIG>. These flags are set to what is known to work for the given compiler vendor, and will thus be enclosed in if-endif clauses that check the CMAKE_<LANG>_COMPILER_ID variable, for example:
if(CMAKE_CXX_COMPILER_ID MATCHES GNU) list(APPEND CMAKE_CXX_FLAGS "-fno-rtti" "-fno-exceptions") ...
