Add a pointer to MacPorts.
Add a pointer to MacPorts.

Thanks to Markus Moll for maintaining the port and pointing me to
it.

Change-Id: Ia28fcbfb4946409de9e6b85d1dc332a25555195b

DougFirErickson/ceres-solver,ysonggit/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,entylop/ceres-solver,entylop/ceres-solver,whuaegeanse/ceres-solver,whuaegeanse/ceres-solver,ysonggit/ceres-solver,whuaegeanse/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,ysonggit/ceres-solver,entylop/ceres-solver,entylop/ceres-solver