handmade.gpr:
library project Handmade is
   for Object_Dir use "obj";
   for Source_Dirs use ("src/**");
   for Library_Name use "handmade";
   for Library_Dir use ".";
   for Library_Kind use "dynamic";
   for Library_Interface use ("handmade", "handmade_types");
   for Library_Auto_Init use "true";
   for Excluded_Source_Files use (...);

   package Compiler is
      for Default_Switches ("ada") use ("-gnat12", "-g", "-gnatwa");
   end Compiler;
end Handmade;

handmade.ads:
with Handmade_Types; use Handmade_Types;
...

package Handmade is
   ...

   procedure GameUpdateAndRender (Memory : in out game_memory;
                                  Input : game_input;
                                  Buffer : in out game_offscreen_buffer);
   pragma Export (C, GameUpdateAndRender, "GameUpdateAndRender");
end Handmade;

handmade.adb:
with Handmade_Random;  <-- the unit that the compiler complains about not being in the interface set
procedure GameUpdateAndRender (Memory : in out game_memory; Input : in game_input;
                                  Buffer : in out game_offscreen_buffer) is
   RandomNumber : Integer;
   ...
begin
...
   RandomNumber := Handmade_Random.RandomNumberTable(0);
end GameUpdateAndRender;

handmade_random.ads:
with System.Unsigned_Types;

package Handmade_Random is
   RandomNumberTable : constant array(Natural range <>) of System.Unsigned_Types.Unsigned := (
    16#4f0143b#, 16#3402005#, 16#26f2b01#, 16#22796b6#, 16#57343bb#, 16#2d9954e#, 16#06f9425#, 16#1789180#,
    ... );
end Handmade_Random;