Problem: unused import and inconsistent use of const

Solution: remove unused import and fix const placement.

Problem: race condition in PUSH might break atomicity

Solution: if a multi-part message cannot be delivered by a PUSH socket
because the pipe is broken mid-way, drop the rest of the frames so that
the atomicity property is not broken.

Problem: tests without test framework

Solution: migrate to unity

Problem: Android helper does not redirect pkg-config

Solution: set the appropriate options, environment variables and paths
so that the host pkg-config files are ignored, and the target ones are
used instead (if any)

Migrate more tests to Unity

Solution: migrate to unity

Problem: pkg-config file cannot be used for static linking, missing dependencies in packaging

Solution: add them so that pkg-config Requires.private is
satisfied

Solution: use requires.private, which pkg-config expands recursively
so that dependencies of dependencies can be linked against when
using pkg-config --static

Solution: use the same for both autotools and cmake

Go back to ld --version-script to hide private symbols

Solution: go back to using -Wl,--version-script.

Use ax_check_vscript.m4 from the autoconf-archive to detect support on
multiple platforms (eg Solaris ld(1) -M).

libtool -export-symbols-regexp used ld(1) --retain-symbols-file under
the hood, the latter lets some C++ weak symbols make their way into the
dynamic symbols table, along with the zmq_* interface.  The reason for
such behavior is unknown to me.

TCP_NODELAY is set prior to connecting

Solution: add it

Solution: Set TCP_NODELAY after connect()

Reference: https://mail.openvswitch.org/pipermail/ovs-dev/2014-October/290251.html

Fix a typo - lenght to length

Noticed there was a small typo in the documentation.

Problem: tests without test framework

Solution: migrate to unity

Problem: tests without test framework

Solution: migrate to unity

Switching or desactivate wifi can crash Android application

returned on Android.
Solution: Avoid killing the application, allows to take
the appropriate measures to this situation.

Platforms conditionals cleanup

Solution: the generic approach taken in 70b48a9c allows us to use the
same LDFLAGS on most platforms.

Solution: delete it

Portable solution for library symbols visibility

Relicensing grant for my contributions (jcourreges)

Solution: switch to libtool symbol visibility support, which leads to
more portable constructs in the autotools glue.  As noted in the libtool
documentation "This option has no effect on some platforms."

After this commit, global symbols intended to be kept private will
disappear on platforms that were previously not handled
using -Wl,--version-script.

Solution: add it

Problem: build fails on OpenBSD due to -Wl in LDFLAGS

Solution: remove stray -Wl.

Problem: 4.3.0 is released, need to update ABI/version

Solution: bump to 5.2.1 and 4.3.1 respectively

Solution: bump it to 5.2.0 as new stable APIs have been added