looks like the module can be put into $builddir/`cat pybuilddir.txt`. However pybuilddir.txt is written/computed by the just built python. So either

 - implement distutils.util.get_platform in configure.ac

 - or write a temporary pybuilddir.txt, call the just built
   python -c 'print("build/lib.%s-%" % (distutils.util.get_platform(), sys.version[:3]))' to write the final pybuilddir.txt, move the _sysconfigdata.py to this location.
   and remove the pybuilddir.txt writing in setup.py