For OS X detect (based on gcc version) and use -single_module flag.

For OS X detect (based on gcc version) and use -single_module flag.
Also removed the use of the flat namespace flags to prevent symbol
name conflicts when an app loads other dynlibs (that may also contain
libpng for example.)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@23556 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775