It seems to have been accidentally removed in the massive modularization commit (0c1471f1).  Removed check for 10.4 SDK since VTK6 now requires 10.5.  VTK_REQUIRED_OBJCXX_FLAGS is set to "-fobjc-gc" so that VTK libraries can be linked into either GC or non-GC processes.

Change-Id: Ifaeb2b4299b2aa4b92fd32bf9abdd5ad96b42ab7