automake version is a bit picky about installation location and uses
different compiler flags from distutils. We prefer official distutils.
Now nghttp2.c, which is generated by cython, is distributed in
archive.
To build extension module, cython is required.
The module is not built with `make` in the top directory.
A user has to run `make build_ext` in python directory.
Currently header compression objects are available for testing.
Tatsuhiro Tsujikawa