Fix compilation with g++ 4.7 (prerelease).

Fix compilation with g++ 4.7 (prerelease).

The workaround for a bug in g++ 3.5 breaks compilation with 4.7 which
implements two-phase lookup correctly, so disable this workaround for 4.7 and
later. We could probably even only enable it for 3.x but this doesn't really
matter as previous 4.x releases don't have problems with this anyhow.

Closes #13385.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68594 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775