Use _MSC_VER (instead of _WIN32) for things that are specific for
Visual Studio.
Also remove #include <winsock2.h> from hiredis.h, as it leaks too
many symbols and defines into the global namespace, which is
undesirable for a public interface header. Anyone who uses the
the affected parts of the hiredis API needs to include the
appropriate headers anyway in order to declare struct timeval
variables.
