The previous commit revealed that Python wasn't actually using
Berkeley DB; it only had it in its closure due to the build-time flag
dump in Makefile and _sysconfigdata.py. When Python detects both GNU
gdbm and Berkeley DB at build time, it will use the former.
This reduces Python's closure size from 200 MiB to 129 MiB. Even
better would be to get move tkinter to a separate output or package
(since that would get rid of all X11 stuff), but that's a bit harder.