Right now, you can't relocate the entire Pango install tree somewhere
else just by setting environment variables... you actually have to
write out an RC file to point to your pango.modules file and then
point to the RC file with PANGO_RC_FILE.

This is pretty sucky. I'll attach a patch that makes things a little
better by adding PANGO_SYSCONFDIR and PANGO_LIBDIR environment variables
(GTK+ weirdly has GTK_SYCONFDIR and GTK_EXE_PREFIX + "/lib, I'd
rather be internally consistent than copy that.)

You might wonder then, how this avoids writing a temporary file. Don't
you still need to write a pango.modules file? It works for me because 
I wanted this on OS X and you can pass @executable_path/../lib/pango/modules/... to dlopen() there, so I put that in the pango.modules. 
But it wouldn't work on most Unix.

It would be possible:

 - Use dladdr() to locate the path to the library to avoid environment
   variables.
 - Implement a generic solution like the Win32 path-rewriting hack
   in find_or_create_module.

And then the Pango install tree could just be relocated on any Unix.
(OTOH, on Linux, people seldom want relocatable trees.)