About Dialogs with header bars have extra buttons
Steps to reproduce
Compile and run aboutdialog.cpp (can someone tell me how to do this in C?)
I don't think that this is specific to Gtkmm, since Builder's about dialog has the same issue.
Current behavior
Expected outcome
Close and credits buttons shouldn't be there.
Version information
Gtk 3.24.8 on arch linux, Gtkmm 3.24.1