I've managed to compile and link it fully by fixing some "bugs" and commenting out some instructions that stop it from compiling and seem to require quite deep investigation.
One problem is that the compiler reports Set_Priorities as undefined e.g.
Set_Priorities (Main_Window.Process_Mdi, (Left, Top, Bottom, Right));
I've searched everywhere but cant find its declaration on my system or any reference on the web.
However, grepping /usr/local produces, in addition to mathes for cc1 etc:
Binary file ./ada-4.3/libexec/gcc/i686-apple-darwin8/4.3.0/gnat1 matches
Binary file ./gnat/libexec/gcc/i686-apple-darwin8/4.3.0/gnat1 matches
(gnat is a link to ada-4.3)
Thanks for any advice,
Roger