Handle CMAKE_OSX_DEPLOYMENT_TARGET for official packages

Because we don't set CMAKE_OSX_DEPLOYMENT_TARGET in any of our CMake projects, the minimum macOS deployment version decided by CMake seems to be the current version on which PySide is built. This means that the 10.12 Packaging config will set deployment target to 10.12, instead of 10.10 (which is the qt base value).

We need to fix to make PySide use the same minimum deployment version as qtbase.