It is an early project decision to use gnu99 & Plan9 C extensions, to
simplify C code.

So far we only build stub wendelin/bigfile/_bigfile.so .

Makefile is introduced because there will be targets which are easier to
handle at make level. For end users no make knowledge is required -
usual `python setup.py build|install|...` work, redirecting to make
where necessary.

As was promised (e870781d "Top-level in-tree import redirector")
setup.py contains install-time hooks to handle in-tree wendelin.py and
install it as a module namespace.

For sdist, we just use `git ls-files` info if we are in a checkout.