Remove configure test for override keyword.

Detect its support in the code by testing __cplusplus value and using specific
checks for MSVC and Clang as configure detects it as being available when
using recent g++ versions in non-C++11 mode, which do support this keyword but
warn when it is used without -std={gnu,c}++11 option, which makes actually
using it a bad idea in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76174 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775