MacOSX 'can't find libzmq' when installed on /usr/local/lib

Fixed by copying a configure block from jzmq ;)

https://github.com/zeromq/czmq/issues/319

With this patch the --with-libzmq= starts working, but I'm pretty sure the --with-libzmq-lib-dir and --with-libzmq-include-dir still doesn't work.

But the more important thing is that now everything (sodium, libzmq, czmq) compiles with a simple ./autogen.sh && ./configure && make && make install