monit: cross-compile, and make openssl optional

Upstream Monit optionally uses OpenSSL to provide TLS support in its
builtin admin web server.  Being able to turn off SSL in Nixpkgs'
monit derivation makes it much easier to build Monit on embedded
systems.

Security implication: if you choose not to build in openssl
then you should probably configure Monit to allow access only from
localhost.