python: restore more compatible behavior for $PYTHON. For https://bugs.gnu.org/74434. * lib/py-compile: if $PYTHON -V does not include the string "python" (case-insensitive), consider the support intentionally disabled and exit successfully, unless PYTHON is set to false, in which case exit unsuccessfully. This is closer to the old behavior. Mention this in the help message. * t/py-compile-env.sh: add test for PYTHON=:. * NEWS: mention this. (And, en passant, add some past bug#s and clarify that only RCS/SCCS pattern rules were disabled, not all.)