Can someone document the PKGCONFIG_OVERRIDE in the Packages.txt?

When should it be used? And an example with explanation on how it can be
used?

The CVS log for bsd.pkg.mk says:
 Add PKGCONFIG_OVERRIDE to get rid of tons of unnecessary patches.
 PKGCONFIG_OVERRIDE may contain a list of ".pc.in" pkgconfig template
 files that require the addition of -Wl,-R in the pre-configure stage.

I see an example for x11/gtk2.

(I am working on a mozilla-gtk that will provide a mozilla-gtkmozembed.pc
and I'd like to know the proper steps for doing this.)

Thank you,

   Jeremy C. Reed
   http://www.reedmedia.net/