Only reject -f(no-)color-diagnostics for known GCC compiler

Only reject -f(no-)color-diagnostics for known GCC compiler

61ce8c44 made it so that ccache rejects -f(no-)color-diagnostics early
for non-Clang compilers. This was needed to avoid false cache hits for
the GCC case, but it had the side effect of rejecting
-fcolor-diagnostics for unknown compilers as well, such as for a
compiler named c++ that in reality is clang++ (which actually accepts
-f(no-)color-diagnostics).

Fix this by simply doing the special case for GCC instead of non-Clang.
This is OK since the speculative handling of color diagnostics options
is only done for GCC and Clang, not for other compiler types.

Fixes #806.