This change finishes fixing https://bugs.gnu.org/30556.
* t/python-prefix.sh: use $PYTHON if set, not hardwiring "python".
* t/python-virtualenv.sh (get-pyexecdir): new make target;
use it instead of hardwiring $py_site.
None of these files are (or can be) executed directly, so drop the
spurious +x permission bit. None of the other tests do this either.
* t/built-sources-install-exec.sh: Remove +x bit.
* t/ccnoco-deps.sh: Likewise.
* t/ccnoco-lib.sh: Likewise.
* t/ccnoco-lt.sh: Likewise.
* t/perf/cond.sh: Likewise.
* t/perf/testsuite-recheck.sh: Likewise.
* t/perf/testsuite-summary.sh: Likewise.
* t/python-prefix.sh: Likewise.
* t/tags-lisp-space.sh: Likewise.
* t/test-extensions-empty.sh: Likewise.
--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.
* maintainer/syntax-checks.mk (toupper): Tighten the regexp
to avoid a new false-match in t/python-prefix.sh.
* t/instmany-python.sh: Use run_make in place of some $MAKE uses.
* t/python-prefix.sh: Likewise.
* t/python-vars.sh: Likewise.
