Adjust configure.ac to hopefully make python PEP3149 compliant filenames

Or at least make a better guess at the library filename extension.
Apply configure.ac updates from python3_compat.patch
Let make build the PEP3149 extended filenames.

This is done using the PYTHON_LIBEXT build system variable
and pulling the full extension from EXT_SUFFIX. It is currently
unclear why EXT_SUFFIX differs from SO on debian python3.3