Solution: establish a matrix of CI options. On one axis we have the
build system (autotools, cmake, android) and on the other axis we
have the encryption options (tweetnacl, libsodium or none).

Solution: use ZMQ_USE_TWEETNACL as a define as expected instead of
HAVE_TWEETNACL

Solution: avoid cloning libsodium, default build is tweetnacl

- they have no copyright / license statement
- they are in some randomish directory structure
- they are a mix of postable and non-portable files
- they do not conform to conditional compile environment

Overall, it makes it rather more work than needed, in build scripts.

Solution: clean up tweetnacl sauce.

- merged code into single tweetnacl.c and .h
- standard copyright header, DJB to AUTHORS
- moved into src/ along with all other source files
- all system and conditional compilation hidden in these files
- thus, they can be compiled and packaged in all cases
- ZMQ_USE_TWEETNACL is set when we're using built-in tweetnacl
- HAVE_LIBSODIUM is set when we're using external libsodium

Solution: add builds/cmake/ci_build.sh and call it from travis.yml

Replace _zmq_COMPILER with CMAKE_VS_PLATFORM_TOOLSET; so the suffix can
be automagically generated for every MSVC compiler supported by cmake.

* report from Richard Newton indicated previous solutions was not
  working on Windows

* was problem on debian/wheezy

The decision about the poller mechanism to use (select, poll, ...)
was done twice: once by the build system and once by the code in
poller.hpp. As the build-system can actually detect the mechanisms
available, prefer that result to the hard coded defaults in
poller.hpp.

At the same time, remove the duplicate detection of select() vs.
poll()-variant from proxy.cpp, signaler.cpp and zmq.cpp.

This patch has not been tested on many build platforms: especially
the cmake build needs testing / patching. For the other builds,
hard code the result as these these are all Windows platforms.

Fix link errors when using -m32 with MinGW

Fix warnings with ICC.

Make doc building option dependent on asciidoc being installed
Fix MSVC build requiring cygwin.
Don't use try_run to get the version