gtk4 fails to build on non eGL platforms

[Martin Husemann <martin%duskware.de@localhost>]

I see this on macppc (running NetBSD-current with pkgsrc-current) when
trying to compile x11/gtk4/work/gtk-4.12.5/gdk/x11/gdkdisplay-x11.c:

[24/1782] Compiling C object gdk/x11/libgdk-x11.a.p/gdkdisplay-x11.c.o
FAILED: gdk/x11/libgdk-x11.a.p/gdkdisplay-x11.c.o 
[...]
In file included from /usr/pkgobj/x11/gtk4/work/.buildlink/include/epoxy/egl.h:46,
                 from ../gdk/x11/gdkdisplay-x11.c:59:
/usr/pkgobj/x11/gtk4/work/.buildlink/include/epoxy/egl_generated.h:11:10: fatal error: EGL/eglplatform.h: No such file or directory
   11 | #include "EGL/eglplatform.h"
      |          ^~~~~~~~~~~~~~~~~~~

Any hints?

Martin