Improvements for building minimal builds and new ports: wxUSE_* usage and minor cleaning.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@39633 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/generic/mdig.cpp

@@ -24,6 +24,8 @@
     #pragma hdrstop
 #endif
 
+#if wxUSE_MDI
+
 #include "wx/generic/mdig.h"
 
 #ifndef WX_PRECOMP
@@ -812,4 +814,6 @@ IMPLEMENT_DYNAMIC_CLASS(wxMDIParentFrame, wxGenericMDIParentFrame)
 IMPLEMENT_DYNAMIC_CLASS(wxMDIChildFrame, wxGenericMDIChildFrame)
 IMPLEMENT_DYNAMIC_CLASS(wxMDIClientWindow, wxGenericMDIClientWindow)
 
-#endif
+#endif // wxUSE_GENERIC_MDI_AS_NATIVE
+
+#endif // wxUSE_MDI