fuzz: fix Makefile default PYTHON_CONFIG_FLAGS to be modern
This is actually what we already do on oss-fuzz, so it's more correct as tests
go.
Differential Revision: https://phab.mercurial-scm.org/D9790
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate