[AD] small dependencies fix

[Elias Pschernig <elias@xxxxxxxxxx>]

Attached is a quick fix for xalleg.h missing in the dependency
generation (already applied it). But all the dep* scripts are just a big
mess, so there may be lots of other problems. I put an item to the todo
list to simply replaces them all with "gcc -M", which will be 100%
reliable, and much simpler.

[And you wouldn't believe how evil wrong dependencies are. I applied the
xor11_2.diff, and the makefile would recompile xwin.c and xvtable.c. But
I didn't check very close, only saw that files were being recompiled,
and assumed everything is ok. But while testing, suddenly some programs
encountered spurious X11 async replies, and would work quite slowly. An
hour-long debugging session later I finally discovered the problem being
the wrong dependencies :P.]

-- 
Elias Pschernig

Index: makefile.in
===================================================================
RCS file: /cvsroot/alleg/allegro/makefile.in,v
retrieving revision 1.69
diff -r1.69 makefile.in
212a213
> xalleg_h = $(srcdir)/include/xalleg.h