doc: Don't install emacs docs when they are not built

doc: Don't install emacs docs when they are not built

In 40b025 we stopped building the notmuch-emacs documentation if
HAVE_EMACS=0 (i.e. no emacs was detected by configure). Unfortunately
we continued to try to install the (non-existent) documentation, which
causes build/install failures.

As a bonus, we also avoid installing the documentation if the user
configures --without-emacs.

Thanks to Ralph Seichter for reporting the problem, and testing
previous versions of this fix.