[jpstewart]

But I haven't managed to avoid these errors about GenericEvent:

Code:

...
Window_X11.c:504:14: error: 'GenericEvent' undeclared (first use in this function)
  504 |   e->type == GenericEvent; /* For XInput events */
...

Have you got any clue ?

GenericEvent is defined in the X.h header from the X.org implementation but not defined anywhere in IRIX. I'm pretty sure that's why the original poster says the libX11-devel package from Sgug-RSE is required.