Enable colored warnings in ninja's output

Summary:
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 commit adds such flag in ecm for gcc >= 4.9 and clang >= 3.5,
and only if the CMAKE_GENERATOR is Ninja.

Test Plan: ninja+gcc and ninja+clang now show nice colored compiler warnings.

Reviewers: Frameworks

Differential Revision: https://phabricator.kde.org/D3733