Darwin: threads, shlibs

I was installing a Python development environment on
my Mac today, and PostgreSQL 7.4.2 as a part of it. I
noticed that according to src/tools/thread/thread_test,
you can add THREAD_SUPPORT=yes to src/template/darwin.

Additionally, PL/Python compiles and works fine
without a libpython dylib, so you can also add
allow_nonpic_in_shlib = yes to
src/makefiles/Makefile.darwin.

Note that I can only test this on Mac OS X 10.3
so if nobody's able to test it on 10.2, you might
need to add a check for Darwin 7.0 or later.
(10.1 probably doesn't work at all currently,
so no worries there...)

The cool thing here is that with these changes,
an out-of-the-box Panther system can compile and use
all of --with-tcl, --with-python and --with-perl.

Marko