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

Solution: mention new version of wepoll and Android Things fix

Solution: fix them

Problem: compiler might not support atomic warning flag

Solution: check before using it

* include atomic when needed

* update for tipc

* moved check under android

* added license

Problem: wepoll 1.5.2 is outdated

Solution: Update to upstream version 1.5.4

Problems: no example for STATISTICS proxy command, no getters for some context options, new events zmq_stopwatch_intermediate proxy STATISTICS and context thread options are eligible to be made STABLE

Solution: do it

Solution: add it

Solution: do it