Don't force color diagnostics outside of ninja

Building with USE_NINJA=false and piping build output to a file will
result in ANSI color codes in the build output.  Only force ANSI codes
on when building with ninja, which strips the ANSI codes when writing
build output to a non-TTY.

Change-Id: Ifd2fb53799fd99c23b7ad91cdb84d943bf707db0
2 files changed