Visual Studio Builds: Don't Generate atk.def

Update the config.h.win32.in template to define _ATK_EXTERN as
__declspec(dllexport) extern for Visual Studio builds, so that the public
symbols and variables can be exported during the build, and generating
atk.def will no longer be needed.  Update the projects and property sheets
accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=728031