fixed compilation - added detection for _WIN32_WINNT

Fix for cmake generator for MSVC2010/2012 and possible others.

Issue: incorrect _WIN32_WINNT detection with cmake script if we using ninja generator and msvc compiler.

Error log:
c:\work\github\zmq.msvc2010>cmake ..\zeromq3-x -GNinja
...
c:\work\github\zmq.msvc2010>ninja

[8/64] Building CXX object CMakeFiles\libzmq.dir\src\clock.cpp.obj
FAILED: "C:/Program Files (x86)/CMake 2.8/bin/cmcldeps.exe" CXX C:\work\github\zeromq3-x\src\clock.cpp "CMakeFiles/libzmq.dir/src/clock.cpp.obj.d" CMakeFiles\libzmq.dir\src\clock.cpp.obj  "Note: including file: "  "c:/Program Files (x86)/Microsoft Visual Studio 10.0/VC/bin/amd64/cl.exe" c:\PROGRA~2\MICROS~2.0\VC\bin\amd64\cl.exe   /nologo /TP /DWIN32 /D_WINDOWS /W3 /GR /EHsc /MP /D_DEBUG /MDd /Zi
/Ob0 /Od /RTC1 -IC:\work\github\zeromq3-x\include -I.  -DDLL_EXPORT -DFD_SETSIZE
=1024 -DWIN32 -D_CRT_SECURE_NO_WARNINGS -D_WIN32_WINNT="" -Dlibzmq_EXPORTS /FoCM
akeFiles\libzmq.dir\src\clock.cpp.obj /Fdlib\libzmq-mt-gd-3_2_4.pdb -c C:\work\github\zeromq3-x\src\clock.cpp

C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\include\sdkddkver.h(221) : fatal error C1017: invalid integer constant expression
 
Also fixed  Issue #101