Move dwz to be only for linux for now.
Merge pull request #1251 from ktf/fix-osx-builds

Move dwz to be only for linux for now.
Merge remote branch 'origin/IB/CMSSW_7_3_X/stable' into IB/CMSSW_7_0_X/stable
Merge remote branch 'origin/IB/CMSSW_7_3_X/gcc491' into IB/CMSSW_7_0_X/stable