Use AM_PATH_PYTHON/PKG_CHECK_MODULES for python bindings

* `AM_PATH_PYTHON` is a much more common idiom for building
  and installing python modules than writing your own.
* It also makes cross-compiling the python bindings possible.
  Previously the `PYTHON_CFLAGS`/`PYTHON_LIBS` would have been
  based on the `--build` python and not the `--host` python.
  By using `pkg-config`, we can always redirect the python-X.Y.pc.