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.