Really only set CMAKE_CXX_FLAGS if empty on GCC
Really only set CMAKE_CXX_FLAGS if empty on GCC

pkuhto/pcl,zavataafnan/pcl-truck,MMiknis/pcl,wgapl/pcl,jakobwilm/pcl,DaikiMaekawa/pcl,drmateo/pcl,jakobwilm/pcl,jeppewalther/kinfu_segmentation,fanxiaochen/mypcltest,RufaelDev/pcc-mp3dg,chenxingzhe/pcl,the-glu/pcl,fanxiaochen/mypcltest,starius/pcl,jeppewalther/kinfu_segmentation,starius/pcl,starius/pcl,3dtof/pcl,v4hn/pcl,shangwuhencc/pcl,DaikiMaekawa/pcl,LZRS/pcl,cascheberg/pcl,damienjadeduff/pcl,soulsheng/pcl,sbec/pcl,srbhprajapati/pcl,stfuchs/pcl,soulsheng/pcl,Tabjones/pcl,chenxingzhe/pcl,fskuka/pcl,3dtof/pcl,lydhr/pcl,closerbibi/pcl,fanxiaochen/mypcltest,raydtang/pcl,KevenRing/vlp,shivmalhotra/pcl,locnx1984/pcl,nh2/pcl,fskuka/pcl,RufaelDev/pcc-mp3dg,krips89/pcl_newfeatures,shyamalschandra/pcl,LZRS/pcl,kanster/pcl,raydtang/pcl,damienjadeduff/pcl,LZRS/pcl,nikste/pcl,zhangxaochen/pcl,kanster/pcl,MMiknis/pcl,wgapl/pcl,stefanbuettner/pcl,shangwuhencc/pcl,nikste/pcl,cascheberg/pcl,soulsheng/pcl,jeppewalther/kinfu_segmentation,starius/pcl,pkuhto/pcl,v4hn/pcl,starius/pcl,srbhprajapati/pcl,stfuchs/pcl,ResByte/pcl,nh2/pcl,KevenRing/vlp,mschoeler/pcl,ipa-rmb/pcl,shyamalschandra/pcl,shyamalschandra/pcl,msalvato/pcl_kinfu_highres,mschoeler/pcl,v4hn/pcl,MMiknis/pcl,raydtang/pcl,lebronzhang/pcl,the-glu/pcl,KevenRing/pcl,locnx1984/pcl,fskuka/pcl,3dtof/pcl,sbec/pcl,MMiknis/pcl,stfuchs/pcl,simonleonard/pcl,mikhail-matrosov/pcl,fskuka/pcl,damienjadeduff/pcl,nh2/pcl,sbec/pcl,raydtang/pcl,krips89/pcl_newfeatures,nikste/pcl,kanster/pcl,pkuhto/pcl,ipa-rmb/pcl,shyamalschandra/pcl,the-glu/pcl,wgapl/pcl,Tabjones/pcl,krips89/pcl_newfeatures,KevenRing/vlp,lydhr/pcl,lebronzhang/pcl,3dtof/pcl,RufaelDev/pcc-mp3dg,drmateo/pcl,ResByte/pcl,fanxiaochen/mypcltest,chenxingzhe/pcl,Tabjones/pcl,lebronzhang/pcl,mschoeler/pcl,kanster/pcl,zhangxaochen/pcl,soulsheng/pcl,drmateo/pcl,shivmalhotra/pcl,chenxingzhe/pcl,closerbibi/pcl,nikste/pcl,cascheberg/pcl,RufaelDev/pcc-mp3dg,Tabjones/pcl,DaikiMaekawa/pcl,nikste/pcl,fanxiaochen/mypcltest,msalvato/pcl_kinfu_highres,simonleonard/pcl,ipa-rmb/pcl,cascheberg/pcl,sbec/pcl,raydtang/pcl,KevenRing/vlp,msalvato/pcl_kinfu_highres,kanster/pcl,msalvato/pcl_kinfu_highres,KevenRing/pcl,stefanbuettner/pcl,MMiknis/pcl,mikhail-matrosov/pcl,DaikiMaekawa/pcl,KevenRing/pcl,LZRS/pcl,stfuchs/pcl,wgapl/pcl,damienjadeduff/pcl,stfuchs/pcl,locnx1984/pcl,KevenRing/pcl,cascheberg/pcl,simonleonard/pcl,v4hn/pcl,shangwuhencc/pcl,shangwuhencc/pcl,KevenRing/pcl,stefanbuettner/pcl,Tabjones/pcl,chatchavan/pcl,drmateo/pcl,pkuhto/pcl,pkuhto/pcl,soulsheng/pcl,fskuka/pcl,ipa-rmb/pcl,zhangxaochen/pcl,mikhail-matrosov/pcl,shivmalhotra/pcl,drmateo/pcl,zavataafnan/pcl-truck,zhangxaochen/pcl,ipa-rmb/pcl,jeppewalther/kinfu_segmentation,jakobwilm/pcl,sbec/pcl,chenxingzhe/pcl,shangwuhencc/pcl,nh2/pcl,lydhr/pcl,simonleonard/pcl,LZRS/pcl,msalvato/pcl_kinfu_highres,chatchavan/pcl,jeppewalther/kinfu_segmentation,stefanbuettner/pcl,v4hn/pcl,zhangxaochen/pcl,the-glu/pcl,3dtof/pcl,locnx1984/pcl,lydhr/pcl,RufaelDev/pcc-mp3dg,zavataafnan/pcl-truck,mikhail-matrosov/pcl,mschoeler/pcl,chatchavan/pcl,krips89/pcl_newfeatures,shivmalhotra/pcl,KevenRing/vlp,krips89/pcl_newfeatures,locnx1984/pcl,shivmalhotra/pcl,lydhr/pcl,closerbibi/pcl,mschoeler/pcl,closerbibi/pcl,ResByte/pcl,jakobwilm/pcl,srbhprajapati/pcl,jakobwilm/pcl,stefanbuettner/pcl,mikhail-matrosov/pcl,shyamalschandra/pcl,zavataafnan/pcl-truck,lebronzhang/pcl,ResByte/pcl,zavataafnan/pcl-truck,damienjadeduff/pcl,srbhprajapati/pcl,simonleonard/pcl,the-glu/pcl,closerbibi/pcl,ResByte/pcl,wgapl/pcl,lebronzhang/pcl,DaikiMaekawa/pcl,chatchavan/pcl,srbhprajapati/pcl