configure: Update python search order

configure: Update python search order

Some systems don't ship with "python" by default anymore, only
"python3" or "python2" or some combination, so include those in the
configure search.

Back-patch of commit 7291733ac.  At the time that was only pushed
back as far as v10, because of concerns about interactions with
commit b21c569ce.  Closer analysis shows that if we just
s/AC_PATH_PROG/AC_PATH_PROGS/, there is no effect on the older
branches' behavior when PYTHON is explicitly specified, so it should
be okay to back-patch: this will not break any configuration that
worked before.  And the need to support platforms with only a
"python3" or "python2" executable is getting ever more urgent.

Original patch by Peter Eisentraut, back-patch analysis by me

Discussion: https://www.postgresql.org/message-id/flat/1457.1543184081%40sss.pgh.pa.us#c9cc1199338fd6a257589c6dcea6cf8d