Problem: ZMQ_PRE_ALLOCATED_FD is too long

Solution: rename socket option (and variables and files) from
pre_allocated_fd to usefd.

CMake and tweetnacl CI

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

Solution: bump CMake required version to 2.8.12 to avoid:

CMake Error at tests/CMakeLists.txt:110 (target_include_directories):
  Unknown CMake command "target_include_directories".

Solution: add builds/tweetnacl/ci_build.sh and add it in travis.yml

Solution: disable -pedantic when building with tweetnacl to avoid
warning about "long long" not existing in ISO C++ 98

fallback on tweetnacl if libsodium is not found and not explicitly requested

allows building with tweetnacl without cmake

Fixes to Windows builds

Solution: fix these.

Solution: at least for vs2015, add vs2015/build.bat to work the
same was as zproject.

Fix a typo in "Add ZMTP heartbeats" changes

Change to detect POSIX Thread priority support properly

Use memcpy instead of assuming option values are aligned

Otherwise, it's undefined behavior. ubsan catches alignment issues in
the libzmq test suite without this.

Refinement of #f4fe375b

Solution: be more explicit in the code, and in the zmq_recv man
page (which is the most unobvious case). Assert if length is not
zero and buffer is nonetheless null.

There's no value in this as the same pattern is repeated in several
places and it's fair to expect people to understand it.

Solution: revert to the old, one-liner style.

It is poor style for test cases to use the internal API (i.e.
libzmq classes or header files), as this code serves the purpose
of teaching developers how to use the library (it doesn't do this
very well, it's an ambition).

Also, including headers in src/ creates problems when compiling.

Solution: remove use of src/macros.hpp.

Solution: update (with correct one from VS2015)

updates for bumped _WIN32_WINNT version with mingw builds

windows.hpp must be included first due to increases _WIN32_WINNT version

Don't call memcpy with 0 size and NULL pointer(s)

added conditional includes for unix / windows

mingw defaults with _WIN32_WINNT as 0x0502 which doesn't define inet_pton(), so add a conditional check

mingw defaults with _WIN32_WINNT as 0x0502 which doesn't define inet_pton(), so add a conditional check

used static_cast<signed int> around WSA_WAIT_FAILED as it is an unsigned implicitly defined as (0xFFFFFFFF ion winbase.h) and causes a comparison error.

removed use of c++11 style initialiser list for 'sockaddr addr { 0 }' and changed it to 'sockaddr addr = { 0 }'

includes windows.hpp if on windows build for mingw build errors from including zmq.h before windows.h

It's undefined behavior, and ubsan flags it.

Problem: CI builds libsodium from dev branch