Problem: NEWS does not mention UDP multicast transport

Solution: add mention and point to doc/zmq_udp.txt

Problem: NEWS out of date

Solution: add an overview of the new socket options, new APIs,
DRAFT mechanism (and DRAFT APIs). And a dedication.

Solution: add release notes from 4.0.x and 4.1.x releases to
capture all important solved bugs.

 Add WinXP compatibility

#define ZMQ_HAVE_WINDOWS_TARGET_XP  disable uncompatible WinAPI

1. Disable call if_indextoname()
2. Emulate windows Condition Variable API in class condition_variable_t with std::condition_variable
 	
This code can be compiled in MSVC 2015 with option "Platform toolset: Visual Studio 2015 - Windows XP (v140_xp)"

Problem: socket option marked as draft for 4.2

Solution: move comment further below to declare new socket
options as stable for the 4.2.0 release.

Problems: need to bump NEWS and libtool version

Solution: bump ABI version fromo 5:0:0 to 6:0:1 since 4.2 is backward
compatible with 4.1, but new symbols are there

Solution: add one, empty for now, fill in after RC release

Problem: CMake build rejects to build with openpgm enabled

Solution: add optional lookup for openpgm via pkg-config

Suppress warning on uninitialized variable use by initializing variable.

Remove an unnecessary while (true)

This block of code will either return -1 or fall through.
The while (true) does nothing.
The braces limit the scope of int rc.

Increment metadata refcount while it's in pending_metadata

Problem: MS VC++ build broken

Solution: try to detect architecture if building with VC++ and
hardcode pointer size accordingly.
Expressions are not allowed inside declspec intrinsics, which
includes other intrinsics.

Problem: pointer union for zmq_msg_t is a hack

Solution: use compiler's alignment attributes instead which is
clearer and less of a hack.
Pointer alignment violations causing crashes on architectures
such as sparc64 and aarch64.
This also avoid triggering ABI checkers as the change is compatible
even though applications that suffer from the bug should rebuild to
take advantage of the fix.

clock_gettime is now defined in macOS 10.12 SDK

Solution: Rename the custom implementation of clock_gettime for macOS to
alt_clock_gettime and wrap all usage in preprocessor macros to only enable the
alternative version when using macOS <= 10.11.

This issue occurs when targeting macOS 10.11 or earlier but using the 10.12
or newer SDK.

Problem: build API incompatible with 4.1

Solution: keep the new --without-docs option, but also keep the old
--without-documentation with an added deprecation warning.
We can then remove it in the next major release, to leave enough time
for users and maintainers to change it without disruptions.

Problem: 2 connects with same sourceip:port to different destip:port fail

Solution: during a connect with a TCP endpoint if a source address is
passed set the SO_REUSEADDR flag on the socket before the bind system
call.
Add unit test to cover this case for both IPv4 and IPv6.

Update INSTALL to note issues with 'make -j check'

Some of the tests cannot be run in parallel; until the Makefile is fixed, note this in INSTALL.

Problem: Does not build in Visual Studio 2013 / Win32

Solution: #include <netioapi.h> and change some IP_ADAPTER_UNICAST_ADDRESS data types

Add EHOSTUNREACH to errno_to_string

performance enhancement

use clock_gettime if there is no instruction to get cpu tick. It will take about 10% performance enhancement in AIX 7.1.