cmake: gcc/clang: enable colour diagnostics in terminal output

When compiler results are piped through a non-terminal (e.g. ninja)
the compiler disables colour diagnostics. Using `-fdiagnostics-color`
forces the compiler to enable colour output. This flag is always
enabled for clang and gcc.

Setting `CONFIG_COMPILER_COLOR_DIAGNOSTICS=n` disables this
feature and reverts to plain diagnostic messages with no formatting.

Signed-off-by: Arvin Farahmand <arvinf@ip-logix.com>