bug#35322: AM_PATH_PYTHON incompatible with Python frameworks on macOS

[Chih-Hsuan Yen]

Hi Automake experts,

On macOS, if CPython is configured with --enable-framework (default
for MacPorts [1]), Python packages should be installed in
${prefix}/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages.
However, AM_PATH_PYTHON gives
pythondir=${prefix}/lib/python3.7/site-packages.

Here's a minimal example configure.ac:

AC_PREREQ(2.57)
AC_INIT([automake-python-test], 0.1)
AM_INIT_AUTOMAKE([foreign])
AM_PATH_PYTHON([2.7])
AC_CONFIG_FILES([Makefile])

The issue appears to be that AM_PATH_PYTHON overwrites the 'base'
variable. If we remove this overwrite like [2], pythondir contains the
correct path.

Related bug report in MacPorts: https://trac.macports.org/ticket/57329

Regards,

Chih-Hsuan Yen

[1] 
https://github.com/macports/macports-ports/blob/834d761210be0d989076000c454a4a1e32209af5/lang/python37/Portfile#L57
[2] 
https://github.com/macports/macports-ports/blob/master/devel/automake/files/patch-issue57329.diff