Add lispdir option for installing mu4e in a custom directory, which what some pre-meson users tended to do with --lispdir. Passing '-Dlispdir=/tmp/foo/bar' causes the mu4e files to be installed in /tmp/foo/bar/mu4e. Should help for issue #2287.
I.e., use meson build -Demacs=/usr/bin/emacs-27.2 Fixes #2286.
The didn't work for quite some time, and would need quite a few updates to even compile with the latest. So remove.