Put a __cplusplus guard around C++-specific tests in wx/defs.h.

This avoids the warning about __cplusplus being undefined in the recently
added wxOVERRIDE logic and make the existing wx_xxx_cast<>s definitions
simpler.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76177 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775