Log Message:
mono: Switch to pkgsrc boehm-gc on SunOS.

The bundled one has issues with newer GCCs, and the pkgsrc one is newer and
optimised, so switching is simpler than trying to retro-fit an old version.

Fix --with-gc, this appears to have always been broken, and our --with-gc=none
argument actually ended up as --with-gc=included, so make that now explicit.

Bump PKGREVISION.