Don't refresh wxStaticText if its label didn't really change

Optimize calling SetLabel() with the same label as the current one, this
seems to happen quite often in practice and results in flicker, so check
for it explicitly.

src/osx/stattext_osx.cpp

@@ -58,6 +58,9 @@ bool wxStaticText::Create( wxWindow *parent,
 
 void wxStaticText::SetLabel(const wxString& label)
 {
+    if ( label == m_labelOrig )
+        return;
+
     m_labelOrig = label;
 
     // middle/end ellipsization is handled by the OS: