build: add 'lispdir' option

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.

meson_options.txt

@@ -29,3 +29,7 @@ option('emacs',
        type: 'string',
        value: 'emacs',
        description: 'name/path of the emacs executable')
+
+option('lispdir',
+       type: 'string',
+       description: 'path under which to install emacs-lisp files')