[MERGE] Allow overriding the PYTHON executable for 'make extensions', and remove *.so files in 'make clean'.

[Tim Penhey]

On Saturday 20 October 2007 16:31:35 Andrew Bennetts wrote:
> This bundle makes it possible to do “make PYTHON=python2.4” to build the
> extensions for a different version of python than the default “python”
> executable.
> 
> It also fixes “make clean” to remove *.so files, and removes the now obsolete
> line that deletes test????.tmp directories (which are now created in the system
> temp directory and usually get cleaned up automatically anyway).
> 
> -Andrew.

If you are going to use a variable PYTHON, use it everywhere in the Makefile.

Tim