setup: detect statfs
statfs is not defined by POSIX but is available in various systems to help
decide filesystem type. Let's detect it and set the macro HAVE_STATFS.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state