build: Do not force diagnostics-color flag

The -fdiagnostics-color flag is only available on GCC >= 4.9, for
older versions this could raise an error in certain circumstances
(such as when using ccache).  Instead, since -fdiagnostic-color=auto
by default  in gcc-4.9, simply set the required environment variable
to the default one if it's undefined.

Based mostly on the systemd commit f44541bc by Michal Schmidt.