Solution:
- Add Doxygen.cfg file to make it simpler for library users and interested
parties to view library internals/externals
- Add README.doxygen.md as an Overview for Doxygen-generated documentation

Notes:
This could later be added into CI for autogenerated documentation that could be
posted to the gh-pages branch.

Problem: unistd.h redundantly included twice

Problem: test_pub_invert_matching missing in autotools build

Solution: fix it

Solution:
- add it to Makefile.am

Problem: tune_tcp_keepalives signature more than 80 chars on a line

Problem: typos and trailing whitespace in code/comments

Solution: fix it

Solution: fix them

Solution: fix them

Solution: fix them

Problem: XPUB test broken since #1566 on Windows

Solution:
- Adjust test_xpub_proxy_unsubscribe_disconnect() to support different
  protocol types
- Exclude the IPC tests on Windows and OpenVMS

H/T: @somdoron

problem: no way to detect in czmq if timers class available

problem: Tests are failing on windows

Problem: assertion in test_xpub_manual is swapped

Solution: check that both pointers passed as arguments to
test_missing_subscription are non-NULL, instead of the opposite.

Problem: XPUB test broken on Windows since #1569

Solution:
- Adjust test_subscriptions() to support different protocol types
- Run TCP and IPC tests everywhere but on Windows and OpenVMS

Fix unresolved external symbols for gather and scatter classes on MSVC 2010-2015.

Fix issue with unresolved external symbols for the gather and scatter classes by adding them to the MSVC 2010, 2012, 2013 and 2015 projects.

Problem: Windows IPC check broken (check_protocol)

Solution: Fix the preprocessor check from || to &&

Problem: LD search path warnings if using Clang

Solution: PR #1906 did not solve this problem properly; subsequent Travis CI
indicated that the issue happens with Clang/LLVM, so make sure to fix the
issue by detecting if Cmake CMake is using Clang for building the tests.

Problem: test_ipc_wildcard is ran on Windows

Solution: move it to the unix-only section of Makefile.am and
tests/CMakeLists.txt since it uses Unix IPC sockets.

Problem: ZMQ_SRCFD test does not work on Windows

Solution: add Winsock specific assertions, since getpeername() should will
return SOCKET_ERROR (-1) and WSAGetLastError() will be set to WSAENOTSOCK

problem: push-pull socket types are not thread safe

Problem: CMake CI does not run make test, and there is no checking for TIPC

Solution: add make test to the cmake/ci_build.sh script

Solution: add macro in ZMQSourceRunChecks.cmake and optionally
include the TIPC sources if the support is available.
More importantly, only run the TIPC tests if the support is there.

Problem: Linker search path warnings on OS X