Enable ccache for faster builds

Problem: add_fd might be called with fd_ == retired_fd

Solution: add assertion

Correct linking for libzmq-static

all calls to target_link_libraries are now at the same location

Fix inclusion of cmake files in dists

with the rest of the CMake modules

[cmake] fix SOVERSION

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Problem: 4.2.3 is out, version(s) are out of date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