Add --with-snapdir configure option

* m4/man-arg-snapdir.m4: New file.
* configure.ac: Call MAN_ARG_SNAPDIR.
* src/man_db.conf.in: Use @snapdir@ rather than hardcoding /snap.
* NEWS.md: Document this.

Signed-off-by: Nikola Forró <nforro@redhat.com>