Enable cmake builds to (optionally) use ninja

For now just switch libgit2 to ninja.  Hopefully
we can switch to ninja by default at some point.

R=binji@chromium.org

Review URL: https://codereview.chromium.org/553973003

git-svn-id: https://naclports.googlecode.com/svn/trunk/src@1647 7dad1e8b-422e-d2af-fbf5-8013b78bd812
1 file changed