Define CFE_AUTOBACKCOLOR ourselves if necessary

Older MinGW cross-compiler versions don't have this symbol in their headers.

src/msw/textctrl.cpp

@@ -76,6 +76,10 @@
     #define CFM_BACKCOLOR 0x04000000
 #endif
 
+#ifndef CFE_AUTOBACKCOLOR
+    #define CFE_AUTOBACKCOLOR 0x04000000
+#endif
+
 #if wxUSE_DRAG_AND_DROP && wxUSE_RICHEDIT
 
 // dummy value used for m_dropTarget, different from any valid pointer value