Clang discards the entire string if it is not used, removing
the ability to read the features from the compiled binary. That
is prevented by using the symbol.
GNU with -O3 also discards the string, so use the string in a
way which is determined by a runtime value (argc) to prevent
it being discarded.
Add a feature test using the compiler macros and the preprocessor to
determine available features.
Add a CMAKE_CXX_COMPILE_FEATURES variable which contains all features
known to the loaded compiler, and a CMAKE_CXX_KNOWN_FEATURES variable
containing all features known to CMake. Add language standard specific
variables for internal use to determine the standard-specific compile
flags to use.
This will be extended to other languages in the future. Follow-up
commits will add features which will be recorded by the feature test.
Stephen Kelly