I don't know why your emacs doesn't require (depend on) xorg-libs. The
  many X-related packages I have installed do require (and depend on)
  xorg-libs.

I haven't tried xorg yet.  What I meant is that the emacs I built with
the base system X does not depend on anything to indicate it.  So I
expect (but have not tried) that if I installed it on a system with
the non-x sets and xorg from pkgsrc that emacs would not work, since
it will be looking for /usr/X11R6/lib/lib*.so.  That seems like a bug.

This is really the same bug as an emacs pkg not working on a system
without any X, but before pkgsrc x you either had gotten around to
installing x or not, and if you install something and it fails you
just add the sets.  Now it is messier.  I suppose this is an another
argument for syspkgs.