Add colored diagnostics support for GCC (>=4.9)

[bchretien]

This is something that was made possible with clang (cf. #297), and since GCC finally provides color support (since 4.9), this may be worth supporting as well.

[bchretien]

Actually, I missed an issue that said that this option needs to be forced with -fcolor-diagnostics (for clang). For GCC:

Support for colorizing diagnostics emitted by GCC has been added. The -fdiagnostics-color=auto will enable it when outputting to terminals, -fdiagnostics-color=always unconditionally. The GCC_COLORS environment variable can be used to customize the colors or disable coloring. If GCC_COLORS variable is present in the environment, the default is -fdiagnostics-color=auto, otherwise -fdiagnostics-color=never.

This only works with ninja if I add -fdiagnostics-color=always, even though GCC_COLORS is set, so I guess the problem is the same (with simply a slightly different compiler flag).

[evmar]

As mentioned there, Ninja knows to strip shell escapes when used via a pipe. It's kind of subtle, I know, but we don't have any better solution... :\

[byron-hawkins]

As mentioned there, Ninja knows to strip shell escapes when used via a pipe. It's kind of subtle, I know, but we don't have any better solution... :\

The solution used by GCC for years is a better solution than what ninja does (always stripping shell escapes for pipes). Please add a standard 3-way color option: { never, auto, always }, where the current behavior corresponds to auto.

Diagnostics are very difficult to read, particulary where C++ templates are involved. Color is fairly essential. If it is not possible to add this option, can you give me a general idea how I can modify my own ninja instance to support this feature?