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/setup.h

@@ -448,17 +448,13 @@
 // Recommended setting: 1 (but may be safely disabled if you don't use it)
 #define wxUSE_SECRETSTORE   1
 
-// Allow the use of the OS built-in spellchecker on wxTextCtrl (multiline
-// only). Currently only wxGTK3 and wxMSW are implemented.
+// Allow the use of the OS built-in spell checker in wxTextCtrl.
 //
-// Default is 1 on wxGTK3 and wxMSW platforms and 0 otherwise.
+// Default is 1, the corresponding wxTextCtrl functions simply won't do
+// anything if the functionality is not supported by the current platform.
 //
-// Recommended setting: 1 on supported platforms.
-#if (defined(__WXGTK3__) || defined(__WXMSW__))
+// Recommended setting: 1 unless you want to save a tiny bit of code.
 #define wxUSE_SPELLCHECK 1
-#else
-#define wxUSE_SPELLCHECK 0
-#endif
 
 // Use wxStandardPaths class which allows to retrieve some standard locations
 // in the file system

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 );