depends: add ability to skip building zeromq

Summary:

Similar to other depends packages, add the ability to skip building
zeromq by passing NO_ZMQ=1.

Backport of core PR16089.

Test Plan:

cd depends
make build-osx NO_ZMQ=1

Check zmq is not built (under x86_64-apple-darwin16/lib there should
be no libzmq.a).

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5620