Don't use GCC extensions to allocate space for debugging frames.

(Here he goes again!  Why do we put up with this?!)
* debug.h (scm_debug_frame): Make the 'vect' member a pointer to
an scm_debug_info structure, not an in-line array of them.  Add
'info' member, to say how many vect elements we've used, for eval
frames.
* eval.c (SCM_CEVAL): Use alloca to allocate space for vect.  Use
a new variable debug_info_end to mark the end of vect, instead of
the address of the 'info' pointer itself.
[DEVAL] (ENTER_APPLY, SCM_CEVAL, SCM_APPLY): Remove casts of
&debug to scm_debug_frame *; debug is a real scm_debug_frame now.
(SCM_APPLY): Explicitly allocate space for debug.vect.
* debug.c (scm_m_start_stack): Same, for vframe.vect.
* stacks.c: Adjusted for new debug frame structure.
(RELOC_INFO, RELOC_FRAME): New macros.
(stack_depth, read_frames): Use them, and new scm_debug_frame
element 'info', instead of magically knowing that eval frames have
an info pointer sitting after vect.
(scm_make_stack, scm_stack_id, scm_last_stack_frame): Use
RELOC_FRAME.
(scm_init_stacks): Formatting tweaks.

libguile/debug.h

@@ -124,7 +124,8 @@ typedef struct scm_debug_frame
 {
   struct scm_debug_frame *prev;
   long status;
-  scm_debug_info vect[1];
+  scm_debug_info *vect;
+  scm_debug_info *info;
 } scm_debug_frame;
 
 #ifndef USE_THREADS