Fix Mac OS X build script - only sed when there is install target.
Fix Mac OS X build script - only sed when there is install target.

Projects using Urho3D as external library may not define any install target, so the attempt to correct CMake generator known issue back fired and caused an error.

helingping/Urho3D,victorholt/Urho3D,urho3d/Urho3D,victorholt/Urho3D,PredatorMF/Urho3D,MonkeyFirst/Urho3D,tommy3/Urho3D,cosmy1/Urho3D,urho3d/Urho3D,codemon66/Urho3D,codedash64/Urho3D,codemon66/Urho3D,weitjong/Urho3D,fire/Urho3D-1,carnalis/Urho3D,weitjong/Urho3D,PredatorMF/Urho3D,SirNate0/Urho3D,henu/Urho3D,kostik1337/Urho3D,bacsmar/Urho3D,299299/Urho3D,MonkeyFirst/Urho3D,codemon66/Urho3D,cosmy1/Urho3D,iainmerrick/Urho3D,c4augustus/Urho3D,MeshGeometry/Urho3D,helingping/Urho3D,kostik1337/Urho3D,carnalis/Urho3D,MeshGeometry/Urho3D,PredatorMF/Urho3D,c4augustus/Urho3D,rokups/Urho3D,fire/Urho3D-1,orefkov/Urho3D,c4augustus/Urho3D,tommy3/Urho3D,cosmy1/Urho3D,urho3d/Urho3D,c4augustus/Urho3D,orefkov/Urho3D,SuperWangKai/Urho3D,abdllhbyrktr/Urho3D,kostik1337/Urho3D,xiliu98/Urho3D,iainmerrick/Urho3D,rokups/Urho3D,299299/Urho3D,MonkeyFirst/Urho3D,iainmerrick/Urho3D,xiliu98/Urho3D,SirNate0/Urho3D,abdllhbyrktr/Urho3D,victorholt/Urho3D,bacsmar/Urho3D,luveti/Urho3D,fire/Urho3D-1,299299/Urho3D,SirNate0/Urho3D,eugeneko/Urho3D,tommy3/Urho3D,tommy3/Urho3D,codedash64/Urho3D,tommy3/Urho3D,henu/Urho3D,luveti/Urho3D,xiliu98/Urho3D,kostik1337/Urho3D,orefkov/Urho3D,299299/Urho3D,henu/Urho3D,fire/Urho3D-1,bacsmar/Urho3D,iainmerrick/Urho3D,luveti/Urho3D,c4augustus/Urho3D,codedash64/Urho3D,luveti/Urho3D,rokups/Urho3D,MeshGeometry/Urho3D,weitjong/Urho3D,weitjong/Urho3D,rokups/Urho3D,xiliu98/Urho3D,helingping/Urho3D,SirNate0/Urho3D,299299/Urho3D,abdllhbyrktr/Urho3D,SuperWangKai/Urho3D,MonkeyFirst/Urho3D,helingping/Urho3D,luveti/Urho3D,henu/Urho3D,weitjong/Urho3D,cosmy1/Urho3D,rokups/Urho3D,SuperWangKai/Urho3D,MeshGeometry/Urho3D,fire/Urho3D-1,abdllhbyrktr/Urho3D,kostik1337/Urho3D,SirNate0/Urho3D,eugeneko/Urho3D,victorholt/Urho3D,henu/Urho3D,carnalis/Urho3D,helingping/Urho3D,abdllhbyrktr/Urho3D,urho3d/Urho3D,MonkeyFirst/Urho3D,codedash64/Urho3D,bacsmar/Urho3D,iainmerrick/Urho3D,eugeneko/Urho3D,xiliu98/Urho3D,codemon66/Urho3D,codedash64/Urho3D,codemon66/Urho3D,MeshGeometry/Urho3D,carnalis/Urho3D,eugeneko/Urho3D,victorholt/Urho3D,SuperWangKai/Urho3D,rokups/Urho3D,cosmy1/Urho3D,orefkov/Urho3D,SuperWangKai/Urho3D,carnalis/Urho3D,299299/Urho3D,PredatorMF/Urho3D