ruby: use in-tree notmuch library

Currently it simply finds any library available, and if notmuch is
installed in the system, it would give priority to that library.

Let's implement our own helper functions to link directly to the local
library, and give priority to the local header file.

Also, add an option to properly check if there are missing symbols.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>