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.

cmake_macosx.sh

@@ -44,7 +44,7 @@ if [ "$1" == "-DIOS=1" -a -e $BUILD/CMakeScripts/XCODE_DEPEND_HELPER.make ]; the
 fi
 
 # Temporary fix: known CMake bug (still exists in 2.8.12)
-if [ "$1" == "-DIOS=1" ]; then
+if [ "$1" == "-DIOS=1" -a -e $BUILD/CMakeScripts/install_postBuildPhase.makeDebug ]; then
     # Due to a bug in the CMake/Xcode generator that prevents iOS targets (library and bundle) to be installed correctly
     # (see http://public.kitware.com/Bug/bug_relationship_graph.php?bug_id=12506&graph=dependency),
     # below temporary fix is required to work around the bug