Problem: cannot use zproto for CZMQ itself

zproto_codec_c was assuming /usr/local/include/czmq.h, which does
not work for classes built as part of CZMQ itself (such as the
zgossip_msg) class.

Solution: check for "../include/czmq.h" and use that preferentially.
Merge pull request #70 from hintjens/master

Problem: cannot use zproto for CZMQ itself