Fix for issue #7085

Fix for issue #7085

On MacOSX 10.6 the CoreFoundation framework must be initialized on the main
thread, the constructor function in that framework will cause an SIGABRT when
it is called on any other thread.

Because a number of extension link (indirectly) to CoreFoundation and the
Python core itself didn't the interpreter crashed when importing some
extensions, such as _locale, on a secondary thread.

This fix ensures that Python is linked to CoreFoundation on OSX, which results
in the CoreFoundation constructor being called when Python is loaded. This
does not require code changes.