Improve -fdiagnostics-color handling

...following up on 13b52f50e52d226c935dcb94fac641c59a77f13f "gbuild: color gcc
output if possible".  Unconditional -fdiagnostics-color=auto overrode the
-fdiagnostics-color=always I specify in CC/CXX to mitigate using the GNU Make -O
option.  Forcing -fdiagnostics-color=always iff gb_COLOR is set should give the
desired results in all cases (colored output to a terminal when calling make
without -O but using a GCC that defaults to -fdiagnostics-color=never, as well
as colored output to a terminal when calling make with -O).

Change-Id: I629bad4bceb15af3ed43f42038825bd1481b33dd
Reviewed-on: https://gerrit.libreoffice.org/84710
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

0 files changed, 0 insertions, 0 deletions