enh: Add usage of DEFINE_validator to test project

test/config/main.cc

@@ -3,9 +3,16 @@
 
 DEFINE_string(message, "Hello World!", "The message to print");
 
+static bool ValidateMessage(const char* flagname, const std::string &message)
+{
+  return !message.empty();
+}
+DEFINE_validator(message, ValidateMessage);
+
 int main(int argc, char **argv)
 {
   gflags::SetUsageMessage("Test CMake configuration of gflags library (gflags-config.cmake)");
+  gflags::SetVersionString("0.1");
   gflags::ParseCommandLineFlags(&argc, &argv, true);
   std::cout << FLAGS_message << std::endl;
   gflags::ShutDownCommandLineFlags();