* mu4e/Makefile: allow configuring the lispdir

`lispdir` is no longer hardcoded in the Makefile.

Users can set the desired `lispdir` at configure time, with `./configure
--with-lispdir='...'`; or at install time with `make lispdir='...' install`.

The default install path is the folder `mu4e/` inside whatever AM_PATH_LISP
finds, which is probably what the original default intended.

This change fixes #320.