Merge branch 'cmake-warnings' of https://github.com/MaartenBent/wxWidgets

Enable warnings for CMake builds and fix some warnings that this
exposed.

Also add wxUSE_NATIVE_DATAVIEWCTRL option and change default GTK version
to 3 for CMake too.

See https://github.com/wxWidgets/wxWidgets/pull/1825

include/wx/generic/private/grid.h

@@ -343,7 +343,7 @@ public:
     {
     }
 
-    virtual bool IsFrozen() const { return true; }
+    virtual bool IsFrozen() const wxOVERRIDE { return true; }
 };
 
 
@@ -375,7 +375,7 @@ public:
     {
     }
 
-    virtual bool IsFrozen() const { return true; }
+    virtual bool IsFrozen() const wxOVERRIDE { return true; }
 };