improving PL/Python builds on OS X

The PL/Python build on OS X is currently hardcoded to use the system
Python install.  If you try to override this when running configure, you
get a mysterious mix-and-match build.  If you want to build against your
own Python build, or MacPorts or Homebrew, you can't.

This is straightforward to fix.  In configure, besides checking Python
include and library paths, we can also check whether it's a framework
build and the right parameters for that.  The attached patch does that
and does the job for me.  Please test it.

One constraint, which is explained in the comment in
src/pl/plpython/Makefile is that in Python <2.5, there is no official
way to detect either framework builds or shared libpython builds, so we
can't support those versions on OS X, at least without more hardcoding
of things.  I'd rather phase some of that out, but if someone needs to
continue to use Python 2.4 or earlier on OS X, let me know.  (Or more
proper fix would be to split DLSUFFIX into two variables, but that seems
more work than it's worth right now.)