horizontal main menu can be activated based on GNUstep-gui ChangeLog:
Add support for horizontal main menu (as in MacOS-X) very heavily
based on the WildMenus bundle by Michael Hanni, but any errors in
the integration are entirely mine (Richard Frith-Macdonald).
Horizontal menus are activated by setting the 'NSMenuInterfaceStyle'
user default to 'NSMacintoshInterfaceStyle'.