Only use cmake28 macro for RHEL6
Only use cmake28 macro for RHEL6

This makes it possible to use the same spec to build on Fedora. It drops any
chance of building on RHEL5, but I doubt that was possible anyway.

Change-Id: Ia956eb6416504e520962ec2f617e03b40ca18203

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