vtk@:6.1.0 : disable Obj-C gc for XCode >= 5.1

Apple removed support for Objective-C garbage collection in binaries
in XCode 5.1, but VTK 6.1.0 and earlier set flags for garbage
collection, resulting in a compile-time bug when compiling VTK using
any recent version of XCode (i.e., Apple Clang).

This commit fixes this bug by removing the garbage collection flags
via setting the variable `VTK_REQUIRED_OBJCXX_FLAGS` to the empty
string. This variable has the same value (i.e., the empty string) in
the root-level CMakeLists.txt file for VTK 6.2.0 and later.