bug#35322: AM_PATH_PYTHON incompatible with Python frameworks on macOS

[Karl Berry]

Hi Joshua and all - regarding https://bugs.gnu.org/35322, I have
(finally) pushed your patches. I tweaked the doc update a little bit and
created a new test to try out the --with-python_prefix and
--with-python_exec_prefix options. (I left the underscores in the name
since I saw there were other such cases already. Confusing either way,
so fine.)

Thanks much for doing all the work. --karl