[notmuch] Notmuch shared library

[Ben Gamari]

Hey all,

Here's a rebased version of my shared library patch. It's pretty similar to the
last version and should build against a clean tree (I'd recommend git clean -fxd).
Let me know if you have any build issues.

In the eyes of the powers that be, what is the long-term status of this patch?
It's been pretty painless to rebase, but it would be nice to merge at some
point. Then perhaps we could focus a bit on the bindings situation. Just a
thought.

Cheers,

- Ben