Revive Amiga port for NetHack 3.7

Update the Amiga Intuition window port (AMII/AMIV) for the 3.7
window_procs API. Key changes:

- Update all window function signatures for 3.7
- Add assembly trampolines for AmigaOS register-based callbacks
- Convert all K&R function definitions to C99
- Add cross-compilation build system (cross-pre1/pre2/post.370)
  using bebbo's m68k-amigaos-gcc with -noixemul -std=gnu17 -m68000
- Clipping fixes: viewport centering, simplified ScrollRaster,
  duplicate Ctrl-R suppression, glyph buffer invalidation
- Add menucolor support in menu rendering
- Move native txt2iff.c and xpm2iff.c to outdated/
- Add nethack.cnf and README.amiga

include/winami.h

@@ -22,6 +22,7 @@ typedef struct amii_mi {
     char gselector;      /* Group selector */
     char canselect;      /* Can user select this entry. */
     char attr;           /* Attribute for the line. */
+    int color;           /* Color for the line (from menucolors). */
     char *str;           /* The text of the item. */
 } amii_menu_item;