test an unsigned int by comparing it with zero

this upstreams a commit from wesnoth, it silences warnings on
some compilers apparently

fd671b1132f1dac983a63d2b625ac5191f84d143

include/spirit_po/po_message.hpp

@@ -32,7 +32,7 @@ struct po_message {
 
   // Check if message is plural. We do this for now by testing msgid_plural.size().
   // Recommended to use this method in case we change it in the future.
-  bool is_plural() const { return static_cast<bool>(id_plural().size()); } 
+  bool is_plural() const { return id_plural().size() > 0; }
 };
 
 /***