Support for C++11 with GCC 4.8.1 and WxWidgets 3.0.1

I've made some changes to be able to compile GD with TDM-GCC 4.8.1 on Windows.
GD is now compiled using -std=gnu++11 and WxWidgets 3.0.1 needs also to be built with that flag (problems with Scintilla if only -std=c++11 is used).
Note : I've fixed the PNG icons because they were causing a lot of warnings with the new WxWidgets 3.0.1 (due to a new version of libpng).