python: do not install in system directories on Darwin 9.

python: do not install in system directories on Darwin 9.

On Darwin 9, get_python_lib returns a path below `/Library/Python'
regardless of the `prefix' argument it was passed, causing `make
install' to target the system directories regardless of `--prefix'
argument used.  Work around this Darwin bug by ignoring the result
of get_python_lib if it points outside of the passed prefix, and
the prefix was not a system directory.

* m4/python.m4 (AM_PATH_PYTHON): If the prefix does not match the
initial portion of the pythondir returned by get_python_lib, then
ignore it unless the configured prefix is `/usr' or starts with
`/System'.  Fixes instmany-python.test failure on Mac OS X 10.5.7.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>