Recognize VC12 (a.k.a. MSVS 2013) and define __VISUALC12__ for it.

Closes #15320.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74479 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775