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

Solution: checkout stable branch instead.
Several warnings are printed when building from the master branch,
and developers recommend using the stable branch instead.

Problem: commit a464eec7 really broke VS2015 builds

Took me over 8 hours to track down the cause of bizarre link
errors when building with libsodium. The VS project files are
not simple things.

Note to self and other maintainers: when someone is obviously
out of their depth, do not merge their changes to build scripts
without cynical appraisal.

Solution: undo the damage.

Problem: Valgrind reports read of freed memory

Solution: when iterating over a map and conditionally deleting
elements, an erased iterator gets invalidated. Call erase using postfix
increment on iterator to avoid using an invalid element in the next
iteration.

Problem: no support for pre-allocated file descriptors [systemd | upstart socket-based activation]

Solution: parse the value set by the ZMQ_PRE_ALLOCATED_FD sockopt
when creating a new TCP socket and use it if valid.
Add new tests/test_pre_allocated_fd_tcp.cpp unit test.