Building GCC-Ada, Syntax error compiling s-excmac.adb with very old gnatmake

I'm trying to update our 20-year old Ada compiler, however you need an Ada-compiler to build some parts of GCC+Ada (9.2.0).

The gnatmake is circa 2000, and it's possibly sub-calling GCC 2.8.1. Sigh.

I have already successfully built several Ada parts of GCC, but this libgnat/ file eludes me:

Compiling libgnat/s-excmac.adb:

$ gnatmake s-excmac.adb
gcc -c -gnatpg s-excmac.adb
s-excmac.ads:134:44: identifier expected
gnatmake: "s-excmac.adb" compilation error

So the problematic line in s-excmac.ads is:

   type GCC_Exception_Access is access all Unwind_Exception;
   --  Pointer to a GCC exception

   procedure Unwind_DeleteException (Excp : not null GCC_Exception_Access );  -- <-- THIS ONE!

   pragma Import (C, Unwind_DeleteException, "_Unwind_DeleteException");

   --  Procedure to free any GCC exception

I don't understand what is wrong with this code. Is there some turn-of-the-century Ada syntax replacement I can use to get this part of GCC-Ada compiled?

I'm a bit of an Ada NOOB, so please be gentle.