python: only use Python's sys.* values if the new option

python: only use Python's sys.* values if the new option

--with-python-sys-prefix is specified; otherwise, return to
previous behavior of using the GNU ${prefix} and ${exec_prefix}.

* doc/automake.texi (Python): document the new behavior.
* m4/python.m4 (AM_PATH_PYTHON): conditionalize use of Python's
sys.* values on the new option --with-python-sys-prefix.
* t/python-prefix.sh: doc update.
* t/python-vars.sh: test both GNU and Python prefix values.
* NEWS: mention this.