Problem: libzmq-dev not installable with libczmq-dev on Ubuntu 16.04

Solution: add alternative dependency as libgnutls-dev | libgnutls28-dev,
so that the resolver can break the tie, as libcurl4-nss-dev depends on
libgnutls-dev which conflicts with libgnutls28-dev

Problem: Android build script is severely outdated

Solution: Migrate build scripts from Android NDK r11c to r20.
- Standalone toolchain
- Migration from GCC to Clang
- Migration from libgnustl to libc++
- Dropped support for API level below 16 (Android 4.1), previously it was API level 9 (Android 2.3)
- Dropped support for mips architecture
- The build script now start the build of all 4 Android architectures (arm, arm64, x86, x86_64)

Problem: build targeting Windows XP is broken

Solution: fix build issues

Revert "Problem: pkg-config file cannot be used for static linking on…

This reverts commit 765c2474.

avoid to use std::ceil

ceil function is usually in math library that can be avoided as dependency

Signed-off-by: Michele Dionisio <michele.dionisio@gmail.com>

Fix linking to libzmq.a statically on CentOS 7

Based on RELICENSE/templates/relicense-template-mplv2.txt. MPLv2 is
definitely fine, but I'm not sure if I'm OK with other licenses.

Solution: add -lm to Libs.private of libzmq.pc so that the std::ceil
usage in src/decoder_allocators.cpp is satisfied during static linking
on CentOS 7.

See https://github.com/zeromq/libzmq/issues/3710 for a reproducer.

Problem: test_wss_transport doesn't compile without DRAFT

Solution: skip it

WIP: problem: appveyor is failing

Problem: packages not build with TLS support

Solution: add VERBOSE=1

Solution: add dependency to gnutls

problem: zero-sized messages cause assertion when glibc assertion are on

solution: add new entries to gitignore

Solution: ignore the line error and continue the build

Solution: fix the test

Solution: when a zero-sized message is encoded or decoded avoid mem copy

problem: GNUTLS handshake is getting blocked because of timeout on some versions

Solution: disable handshake timeout

problem: wss transport return incorrect return code for error

Solution: return -1 for errors

problem: WS transport doesn't support mechanism

Solution: initialize the value

Solution: add capability check to zmq_has

Solution: add support to mechanism

RELICENSE: add Jon Dyte license grant.

Add grant by nixman

Submitted as Github comment:

https://github.com/zeromq/libzmq/pull/242#issuecomment-537903446

Add IBM i build support