Try to avoid multi-threaded operations with Xapian.
This remove the thread workers during indexing, and avoids the indexing
background thread. So, mu4e has to wait once again during indexing.
We can improve upon that, but first we need to know if it avoids the
problem of issue #2756.
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.
