Add support for the RelWithDebInfo CMake configuration.

It's now possible to build Release version with pdb file.

Problem: On Windows CI, in tests/test_timers.c, "Windows.hpp" could not be found.

found.
Solution: Include the source directory

Problem: _sleep is a library function of Windows.
Solution: use sleep_

Problem: Windows CI fails due to libsodium build failure

Solution: Use stable branch of libsodium

problem: when socket is destroyed before zmq_poller is the zmq_poller is accessing a dead socket

Problem: Android build is too slow

Solution: use parallel make (-j 4) and don't build man pages.

Test for issue #1690 (ZMQ_REQ_RELAXED)

Current ZMQ_REQ_RELAXED test improvement to check that pipes are not closed
after executing two send() in a row with no recv() in between.

Fixed issue #1690 (ZMQ_REQ_RELAXED)

When using ZMQ_REQ_RELAXED and a 'send' is executed after another 'send' the
previous code would terminate the 'reply_pipe' if any.
This is incorrect as terminating the reply pipe also terminates the send pipe
as they are the same (a pipe associated with a socket is bidirectional).
Doing a terminate on the pipe sets an internal flag called out_active to false
and the pipe can no longer send messages.
Removing the 'terminate' solves the problem. Removing this call is not an issue
as the incorrect ordering of messages that could be incurred is taken care of
by the ZMQ_REQ_CORRELATE option if needed.

These sockets don't handle multipart data, so if callers send it,
they drop frames, and things break silently.

Solution: if the caller tries to use ZMQ_SNDMORE, return -1 and
set errno to EINVAL.

Problem: still one error in vcxproj files

Solution: fix and try again.

(We could start to think of using zproject to generate these build
scripts as the only difference is C/C++)

Problem: includes timer.cpp/hpp instead of timers.cpp/hpp

Solution: fix it.

Problem: performance tests don't link on Windows/MSVC

Solution: add Iphlpapi.lib to libraries to link against

Problem: MSVC projects don't include timers.cpp/hpp

Fixes for Windows, and poller API

Solution: add these

Solution: change zmq_poller_close(p) to zmq_poller_destroy(&p)

If we're going to add CLASS-like APIs we should use the proper
syntax; specifically 'destroy' instead of 'close', which is a
hangover from the 'ZeroMQ is like sockets' model we're slowly
moving away from.

Solution: change zmq_timers_close(p) to zmq_timers_destroy(&p)

Solution: add to project dependencies

Problem: build scripts don't need full git history

Problem: Windows build gives warning on v2_decoder.cpp:130

Solution: add explicit cast

Solution: add `--depth 1` parameter to `git clone` commands

CMAKE_VS_PLATFORM_TOOLSET may not be set.

E.g. for nmake.

Problem: no way to schedule timers with zmq_poll or zmq_poller 

Add missing header for the if_nametoindex on Windows.

Fix redirecting location of pdb via CMAKE_PDB_OUTPUT_DIRECTORY.