python-2.6.4: make Modules/_ctypes/libffi configure

python-2.6.4: make Modules/_ctypes/libffi configure

This fixes configure issues with recent autoconf, e.g:
  autoreconf: Entering directory `Modules/_ctypes/libffi'
  autoreconf: configure.ac: not using Gettext
  autoreconf: running: aclocal --force
  configure.ac:26: error: m4_copy: won't overwrite defined macro: \
      _AC_ARG_VAR_PRECIOUS
  configure.ac:26: the top level

It also adjustes Makefile.am to take into account the fact that python ships
incomplete source from libffi.

Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Acked-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>