Don't use __declspec(dllexport) for MinGW gcc 4.5 and later.

Don't use __declspec(dllexport) for MinGW gcc 4.5 and later.

Due to the changes in gcc 4.5, copies of dllexported inline functions are now
generated in all object files including their declarations, increasing their
sizes tenfold, and they are also now instantiated in the DLL itself increasing
its size fourfold. Moreover, linking such a gigantic DLL requires inordinate
amounts of memory and takes a very long time, see some statistics at
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43601.

To avoid all these problems, don't use dllexport at all any more but rely on
binutils support for auto export/import which seems to work fine and results
in much smaller DLLs which are created much faster.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67971 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775