Python installation selection in Meson

I noticed that under meson, the selection of the Python installation 
using the 'PYTHON' option doesn't work completely.  The 'PYTHON' option 
determined the Python binary that will be used to call the various build 
support programs.  But it doesn't affect the Python installation used 
for PL/Python.  For that, we need to pass the program determined by the 
'PYTHON' option back into the find_installation() routine of the python 
module.  (Otherwise, find_installation() will just search for an 
installation on its own.)  See attached patch.  I ran this through 
Cirrus, seems to work.