zstd: hack include order to ensure that our zstd.h is found
If the regular Python CFLAGS include directories that already have the
zstd headers available, a different and possible incompatible version
can be picked up otherwise. Sadly, it seems like Python has no easy way
to prefix flags before the rest.
Set up
$ hg init repo
$ cd repo
$ echo a > a
$ hg add a
$ hg commit -m a
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