ENH: allow libs with no sources but complain

Source/cmAddLibraryCommand.cxx

@@ -60,8 +60,11 @@ bool cmAddLibraryCommand::InitialPass(std::vector<std::string> const& args)
 
   if (s == args.end())
     {
-    this->SetError("called with incorrect number of arguments, no sources provided");
-    return false;
+    std::string msg = "You have called ADD_LIBRARY for library ";
+    msg += args[0];
+    msg += " without any source files. This typically indicates a problem ";
+    msg += "with your CMakeLists.txt file";
+    cmSystemTools::Message(msg.c_str() ,"Warning");
     }
 
   std::vector<std::string> srclists;