issue with `diff -D`

[Igor]

Ehllo.

I was trying to use `diff -D` to merge my multilib headers into one 
that would "#ifdef __x86_64__" everywere maintaining things correct. 
However i found this:

file a.h
   --- file a.h ---
   /* some comment
    */
   
   int somefunc();
   --- end of file a.h ---

file b.h
   --- file b.h ---
   /* some DIFFERENT comment
    */

   int somefunc();
   --- end of file b.h ---

If I try to `diff -D __x86_64__ a.h b.h > c.h` i get this:

file c.h
   --- file c.h ---
   #ifndef __x86_64___
   /* some comment
   #else /* __x86_64__ */
   /* some DIFFERENT comment
   #endif /* __x86_64__ */
    */

   int somefunc();
   --- end of file c.h ---

getting a unclosed "#ifndef __x86_64__" messing stuff.

I don't know if this is a bug or if stuff was made simple since diff is 
not a C parser, however, reported.

Thanks.