po4a: fix build on Solarish.

- Conditionally depend on diffutils (borrowed from pkgdiff)
- On "SunOS", resolve /dev/null symlink (if any) during pre-configure
- When invoking gettext-tools, make sure they're the pkgsrc ones
- Add 'sgml' option (also off by default) as suggested by upstream

Fixes build (and passes nearly all tests, with both options enabled) on
Tribblix and Solaris 11, which is exactly as good as things were -- and
are -- on NetBSD.

Bump PKGREVISION.

(schmonz)