build-sys: don't enable color gcc on dumb terminals

build-sys: don't enable color gcc on dumb terminals

Guys, we know that emacs is the best editor on earth, but unfortunately
its "M-x compile" terminal cannot do colors (well, it does its own
highlighting of the output anyway), and it will inform the programs it
calls about this with TERM=dumb, and gcc should check for that. But you
guys turned that off. Not cool. Let's turn it on again.