Log Message:
Fix a MAKE_JOBS safety problem. In the recipe for Python/importlib.h it
launches a submake for _freeze_importlib before all the object files have
been compiled. List $(LIBRARY_OBJS_OMIT_FROZEN) as dependencies to make it
wait until it is safe to launch the submake.

Should fix occasional errors of the sort:
Python/Python-ast.o: file not recognized: File truncated
*** [Modules/_freeze_importlib] Error code 1