zmq.hpp not present in zeromq-4.1; crash during test-stage

Hi. on Slackware 14.2 libopenshot-0.1.2 compilation failed during testing stage (libmagick-related it seemed but not sure).

A note for other people trying this on Slackware:

The recent stable release of zeromq (4.1.5) does not have the header file calling C++ bindings (/usr/include/zmq.hpp) libopenshot calls for (in ZeromgLogger). Ubuntu and Archlinux packages for zmq-4.1.5 include this file (https://raw.githubusercontent.com/zeromq/cppzmq/014628c/zmq.hpp) as a separate add-on (e.g. see https://git.archlinux.org/svntogit/community.git/tree/trunk/PKGBUILD?h=packages/zeromq). Mimicking this for the slackware-package made compilation with cmake possible.