Avoid direct use of std::stringstream

In method cmStringCommand::HandleFindCommand added by parent commit use
the cmOStringStream compatibility wrapper instead of std::stringstream.

Source/cmStringCommand.cxx

@@ -553,7 +553,7 @@ bool cmStringCommand::HandleFindCommand(std::vector<std::string> const&
     }
   if(std::string::npos != pos)
     {
-    std::stringstream s;
+    cmOStringStream s;
     s << pos;
     this->Makefile->AddDefinition(outvar.c_str(), s.str().c_str());
     return true;