Problem: ZMQ_BUILD_DRAFT_API define is not propagated to dependent CMake projects

[boscosiu]

Solution: Add the definition to the relevant CMake targets in public scope. Fixes #4194

[bluca]

Build fails:

C:\projects\build_libzmq\platform.hpp(74,1): error C2220: the following warning is treated as an error [C:\projects\build_libzmq\libzmq.vcxproj]
C:\projects\build_libzmq\platform.hpp(74,1): warning C4005: 'ZMQ_BUILD_DRAFT_API': macro redefinition [C:\projects\build_libzmq\libzmq.vcxproj]
C:\projects\libzmq\src\precompiled.cpp : message : see previous definition of 'ZMQ_BUILD_DRAFT_API' 

[boscosiu]

Thanks, sorry about that. I refactored this to have the build targets receive the definition directly from CMake instead of through the CMake-generated platform.hpp.