Clean up unused variable usage

Unused variables in function calls can be commented out.  Unused
return variables get a new routine `ignore_unused()` that silences the
warnings with zero overhead.

pcbnew/python/scripting/pcbnew_scripting_helpers.cpp

@@ -40,6 +40,7 @@
 #include <drc/drc_engine.h>
 #include <drc/drc_item.h>
 #include <fp_lib_table.h>
+#include <ignore.h>
 #include <io_mgr.h>
 #include <string_utils.h>
 #include <macros.h>
@@ -99,7 +100,7 @@ SETTINGS_MANAGER* GetSettingsManager()
         else
         {
             // Ensure wx system settings stuff is available
-            static_cast<void>( wxTheApp );
+            ignore_unused( wxTheApp );
             s_SettingsManager = new SETTINGS_MANAGER( true );
         }
     }