removing hard-coded path of librt

* Changed CMake tests to use check_symbol_exists in network-related libraries on win32 instead of check_library_exists

prevent duplicate connections from PUB sockets also 

prevent duplicate connections from PUB sockets also (see https://github.com/zeromq/libzmq/issues/788)

Problem: NuGet package versioning out of sync with releases.

Problem: vc++ 11.0 (vs 2012) compile fails on move semantics.

Problem: fn pointers are not bool (vc++ warning C4550).

Problem: Build is broken on OpenBSD

Solution: re-instate block for OpenBSD erroneously removed in 9622a830

Problem: 4.2.3 is out, version(s) are out of date

add ZeroMQConfig.cmake.in in EXTRA_DIST

Solution: bump changelog, ABI revision, library and packaging version

Solution: add it

Problem: nuget packaging refers to imatix gsl

Solution: re-point to the maintained zeromq fork
Signed-off-by: Jim Klimov <EvgenyKlimov@eaton.com>

Problem: Jenkinsfile uses "any" agent

Solution: avoid using infrastructure and default agents by requiring a reasonable label (like in GSL and zproject)
Signed-off-by: Jim Klimov <EvgenyKlimov@eaton.com>

export ZMQ_STATIC compile flag to depending projects (MSVC)

Signed-off-by: Gunther Laure <glaure@dewetron.com>

Problem: static build broken on Win + CMake

Solution: revert the objects optimisation, and go back to building
everything twice on Windows, as the static builds needs different
preprocessor definitions from the shared one, so the objects have to be
rebuilt.
Keep the optimisation for all the other platforms.
Fixes #2858

Problem: Jenkinsfile does not pass distcheck etc.

Problem: WIN32 CMAKECONFIG_INSTALL_DIR is broken

Solution: set it to CMake instead of a subfolder of share.
See cmake find_package documentation for further info.

Problem: Debian packages are missing zmq.hpp

Problem: No Jenkinsfile