cmake: compiler: Check -fdiagnostics-color=always before using

The -fdiagnostics-color flag is only on GCC versions 4.9.0 and newer.
Previously, CMake only checked that the compiler was not the XCC compiler
before using the flag.

However, given that the flag us unavailable on compilers other than XCC,
e.g. old versions of GCC, this patch replaces the previous logic with a
flag-check using the check_set_compiler_property() CMake function.

Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>