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.

configure.ac

@@ -133,7 +133,7 @@ CC_CHECK_FLAGS_APPEND([with_cflags], [CFLAGS], [\
         -ffast-math \
         -fno-common \
         -fdiagnostics-show-option \
-        -fdiagnostics-color \
+        -fdiagnostics-color=auto \
         -fno-strict-aliasing \
         -fvisibility=hidden \
         -ffunction-sections \