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.
