Always set wxUSE_SPELLCHECK to 1 by default for simplicity

It does no real harm to set it to 1 and this avoids the need for the
platform checks in setup.h files, simplifying things (although it does
require checking for GTK 3 in the code compiled for both GTK 2 and 3, as
gtkspell library can only be used with GTK 3).

include/wx/gtk/textctrl.h

@@ -96,13 +96,13 @@ public:
     // Overridden wxWindow methods
     virtual void SetWindowStyleFlag( long style ) wxOVERRIDE;
 
-#if wxUSE_SPELLCHECK
+#if wxUSE_SPELLCHECK && defined(__WXGTK3__)
     // Use native spelling and grammar checking functions.
     virtual bool EnableProofCheck(bool WXUNUSED(enable) = true,
                                   const wxTextProofOptions& WXUNUSED(options) =
                                   wxTextProofOptions()) wxOVERRIDE;
     virtual bool IsProofCheckEnabled() const wxOVERRIDE;
-#endif // wxUSE_SPELLCHECK
+#endif // wxUSE_SPELLCHECK && __WXGTK3__
 
     // Implementation from now on
     void OnDropFiles( wxDropFilesEvent &event );