BUG #6633: PL/Python build does not honor PYTHON on Mac OS X

The following bug has been logged on the website:

Bug reference:      6633
Logged by:          Clay McClure
Email address:      clay@daemons.net
PostgreSQL version: 9.1.3
Operating system:   Darwin / Mac OS X 10.7.3 (Lion)
Description:=20=20=20=20=20=20=20=20

According to the installation documentation, one can set the PYTHON
environment variable to control which python is used to build PL/Python, but
this does not work correctly on Mac OS X.

The configure script correctly sets python_libspec based on the PYTHON
environment variable, but the plpython Makefile clobbers that value:

# Darwin (OS X) has its own ideas about how to do this.
ifeq ($(PORTNAME), darwin)
shared_libpython =3D yes
override python_libspec =3D -framework Python
override python_additional_libs =3D
endif

According to a Homebrew bug report
(https://github.com/mxcl/homebrew/issues/11162):

"You should submit a bug report to PostgreSQL as they can't assume Python is
always built as a Framework on Darwin. They should be using python-config to
get the gory details."