I suppose for my purposes, and for pkgsrc in general, it's probably best
  to simply patch the libpangoxft.pc file so that pkg-config gives the
  full list of dependencies.....

I think this is just a pango bug in generating its .pc file.  I've
written .pc.in files, and included all the libs that the library being
built is linked against.

This is all messy, but pkg-config done right seems sane.  Of course,
that's not the question on the table for pkgsrc.   So fixing up the
.pc file seems like the right thing.

<don-asbestos>
tiff obviously needs a .pc file, so users don't have to know about
-lm.  really.
</>

  The bigger question though is whether or not pkg-config should get its
  list of dependent libraries from the libtool.la file(s) where possible,
  or not.

Perhaps, but pkg-config seems pretty based on reading .pc files, and
no more, at runtime.  I can see how using .la files to link with at
library build time can lead to this desire, however.  I don't want to
discourage you from fixing pkg-config in this direction, but that's
out of scope for pkgsrc.

-- 
        Greg Troxel <gdt@ir.bbn.com>