Bug: Attempt to install notmuch-emacs.info during --without-emacs build

[Ralph Seichter]

* David Bremner:

> The following seems to fix the problem, but I haven't tested it
> extensively.

As shown in https://monksofcool.net/tmp/6bmjUFL.log , your suggestion
results in notmuch-emacs.info being generated but unfortunately not
installed when Notmuch is built --with-emacs. Obviously that's better
than a build failure for --without-emacs, but it is not quite ideal
either. ;-)

-Ralph