[Bug driver/80271] Support environment variable CLICOLOR_FORCE to enable -fdiagnostics-color

[jhasse at bixense dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80271

--- Comment #2 from Jan Niklas Hasse <jhasse at bixense dot com> ---
I guess that's because one doesn't want the build output to depend on an
environment variable (most build systems won't rebuild on env variable
changes)?

-fdiagnostics-color doesn't affect the build output, therefore it would be a
perfect fit for an env variable. Currently adding it causes an unnecessary full
rebuild.