build: Don't hardcode the path of python3
Python is not guaranteed to be installed in /usr/bin. This is especially true for *BSD systems which don't consider Python as an integral part of their systems.
This fixes build on FreeBSD:
gmake: Entering directory '/home/lantw44/gnome/build/mutter/src' GEN meta-enum-types.c GEN meta-default-modes.h /bin/sh: /home/lantw44/gnome/source/mutter/src/backends/native/gen-default-modes.py: not found