Change --with-czmq to --with-libczmq

The help message from ./configure --help says to use --with-czmq, but
this didn't work. Upon checking the generated configure script, it
looked like it was expecting --with-libczmq, so I changed the help
message in configure.in accordingly
Merge pull request #17 from zeromq/topic/fix-configure-with-libczmq

Change --with-czmq to --with-libczmq