CMake: turn on -fdiagnostics-color for Ninja builds

This PR attempts to turn on `-fdiagnostics-color` by default when using
the CMake Ninja generator.

Ninja effectively turns off colored messages by not running subcommands
in a pseudo-terminal. The Ninja maintainers have decided that using
pseudo-terminals has more downsides than upsides, but have helpfully
taught Ninja itself how to parse and remove color codes if its parent
isn't a (pseudo-)terminal.

Colored messages make it easier to read warnings and error messages in a
large build log, and are pretty.

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