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Problem: wepoll 1.5.2 is outdated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

Solution: add one so that class-based bindings can easily use them

Solution: do it

Solution: do it

Problem: radix tree never used in autotools builds

Solution: add a switch and a default to autoconf

* Add specific option to select radix tree implementation for subscriptions (defaults to ON if draft API enabled).

Problems: some tests still use hard-coded TCP ports, wrong usage of CPPFLAGS, missing NEWS, no way to force C++98 builds and CI for it

Problem: C4 process is not easily discoverable.

Solution: Add a relicensing grant.

Solution: Add 'Contributing' section to README.md

Solution: cast the unused variable

Solution: if the compiler supports it, pass C++98-compat flags.
Currently Clang supports this flag but GCC does not.

Add a new flag to enable it, as building with C++98-compat but also
with -std=gnu++11 will cause a lot of warnings due to the backward
compat ifdefs.

Add a CI job to run it and ensure we don't break compatibility.