Can we make X11 optional for libvips?

[atomicules <base%atomicules.co.uk@localhost>]

Various people kindly picked up my long-running WIP libvips package and
made improvements to it and I've just found out it's been moved from
WIP to Pkgsrc - thanks! However, the very reason I first added this
to WIP is because I wanted a lighter weight alternative to ImageMagick
that didn't require X11. I notice that libvips now needs X11 because
of `.include·"../../print/poppler/buildlink3.mk`. Can I/we move this
to `options.mk`?

_Fwiw, I've been without local NetBSD hardware for about three years
(although my cloud server has continued running NetBSD). Hoping that
now I've finally got a local machine running NetBSD again I can be a bit
more active_