python-cffi: experiment with extracting cdefs from notmuch.h

[Floris Bruynooghe <flub@devork.be>]

Hi,

This is mostly an experiment because I got tired of copying the
C function defs from notmuch.h (in other words this was yak shaving
while I intended to see if I could get notmuch_database_get_directory
supported).  I'm not convinced this is better that what we had before
to be honest, but am curious what other people think of this approach.
In deltachat we do actually use this mechanism and it works nicely,
but there we don't have to deal with deprecated functions which is
what makes this ugly here.

Independenly maybe the changes to the makefile and README are useful
anyway.  I don't think the way this was build before always used the
right lib version, I think for running T391 we probably want to
compile with an RPATH.

Cheers,
Floris