I'd very much like to see this upstream.

Here's my updated series for this.

I amended your first patch to make it install the notmuch.h header too.

The second patch fixed some parallel make issue I had while testing your series. We discussed this
ages ago on #notmuch, I don't remember the details at the moment, and I can't check my logs right
now. Any reason why this isn't included in your current patch?

The third patch adds a configure switch to configure where to install libraries. It has one issue
I'm aware off. Setting --prefix=/usr without setting --libdir does the wrong thing. This should
probably be fixed, but I don't have any immediate ideas how to do this.

So, series tested by me and Michael Forney, and I'm using it for my notmuch, please apply. :)