support GCC's -fdiagnostics-color , by Clang only for now

GCC apparently made the option named backwards and different just for the fun
of it. Clang supports also the GCC way.
change code to support color diagnostics also for GCC

Since it's new in GCC 4.8, for now require ICECC_GCC_COLORS set
to actually make it work like it would automatically with Clang
(icecc doesn't know what compiler version it's run with), the default
can be flipped somewhen later.