[PATCH] lib: add library version check macro

[David Bremner]

Jani Nikula <jani at nikula.org> writes:

> There have been some plans for making build incompatible changes to
> the library API. This is inconvenient, but it is much more so without
> a way to easily conditional build against multiple versions of
> notmuch.
>
> The macro has been lifted from glib.

This seems fine to me. I even thought about the licensing for these 3
lines of code ;).

d