Colored compiler warnings in ninja output only work with the
-fdiagnostics-color=always flag.
See https://github.com/ninja-build/ninja/issues/814 for a rationale.
This patch adds such flag in ecm for gcc >= 4.9 and clang >= 3.5;
I'm not sure which version of clang is the first that supports this flag, but it
cannot be < 3.5 because 3.5 was the first release after gcc 4.9.