Check GCC Version before adding -fast compiler option on OSX.
Check GCC Version before adding -fast compiler option on OSX.

-fast compiler option is only supported using Apple's GCC packaged with XCode.
Other GCC versions will fail when this flag is enabled. This commit checks the
GCC version on OSX and only enables this flag when < 4.3. Apple's GCC is
currently 4.2.1 and a user is unlikely to install a non-apple version this old
on OSX.

Change-Id: Ifca9149625c065cd16559d7e30c218a322cf79aa

kartikkumar/ceres-solver,DougFirErickson/ceres-solver,kartikkumar/ceres-solver,DougFirErickson/ceres-solver,timfel/ceres-bin,jmwalls/ceres-solver-fork,whuaegeanse/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,DougFirErickson/ceres-solver,DougFirErickson/ceres-solver,ysonggit/ceres-solver,timfel/ceres-bin,timfel/ceres-bin,jmwalls/ceres-solver-fork,entylop/ceres-solver,entylop/ceres-solver,ysonggit/ceres-solver,ikoruk/ceres-solver,timfel/ceres-bin,ikoruk/ceres-solver,whuaegeanse/ceres-solver,ikoruk/ceres-solver,ysonggit/ceres-solver,ikoruk/ceres-solver,whuaegeanse/ceres-solver,stonier/ceres_solver,jmwalls/ceres-solver-fork,kartikkumar/ceres-solver,kartikkumar/ceres-solver,entylop/ceres-solver,ikoruk/ceres-solver,whuaegeanse/ceres-solver,stonier/ceres_solver,stonier/ceres_solver,stonier/ceres_solver,jmwalls/ceres-solver-fork