backported SF patch #752538 to implement wxGetOSDirectory() for Mac

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_2_4_BRANCH@21048 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/filefn.cpp

@@ -1595,6 +1595,8 @@ wxString wxGetOSDirectory()
     wxChar buf[256];
     GetWindowsDirectory(buf, 256);
     return wxString(buf);
+#elif defined(__WXMAC__)
+    return wxMacFindFolder(kOnSystemDisk, 'macs', false);
 #else
     return wxEmptyString;
 #endif