Pass -fdiagnostics-color=auto to the compiler even if stderr is redirected

This fixes a problem when e.g. a configure test probes if the compiler
(wrapped via ccache) supports -fdiagnostics-color=auto. If ccache discards
the option since configure redirects stderr, then configure will
erroneously think that the compiler supports the option and the build will
fail later on (if stderr refers to a TTY).
test: Unset GCC_COLORS to avoid confusing argument processing tests