* m4/python.m4 (AM_PATH_PYTHON): Check all known Python interpreters

* m4/python.m4 (AM_PATH_PYTHON): Check all known Python interpreters
in loop until we find one the satisfies the user supplied version.
Add python2 to the list of known interpreters.  Don't use
changequote.  Cache the calculation of PYTHON_VERSION and
PYTHON_PLATFORM.
(AM_PYTHON_CHECK_VERSION): New function, extracted from
AM_PATH_PYTHON and modernized.
* m4/runlog.m4: New file.
* m4/Makefile.am (m4data_DATA): Add it.