mu embeds a "vendored" subset of the libfmt package under thirdparty; but it's
better to use the system-package if the user has one.
So, use the system package if found or user -Duse-embedded-fmt=true is
specified.
Single-threaded is the build-default, and seems to work well enough for
1.12.7, so remove the option to turn it off.
This is because build-options that influence such low-level/core
behavior are a pain to maintain.
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.
