Make sure LIBCZMQ_EXPORTS is defined for all DLL configurations

And make sure LIBZMQ_EXPORTS and DLL_EXPORT are not defined for static
configurations.

This allows the ReleaseDLL configuration to build against a release zeromq
library.
Merge pull request #142 from vortechs2000/more_dll_cleanup

Make sure LIBCZMQ_EXPORTS is defined for all DLL configurations