Work around the fact that autosh..tools does only check for header presence, not usability.

src/misc/objects.c

@@ -1482,7 +1482,7 @@ static void ListChildren( vlc_list_t *p_list, vlc_object_t *p_this, int i_type )
 }
 
 #ifndef NDEBUG
-# ifdef HAVE_EXECINFO_H
+# if defined(HAVE_EXECINFO_H) && defined(HAVE_BACKTRACE)
 #  include <execinfo.h>
 # endif
 

src/misc/threads.c

@@ -83,7 +83,7 @@ static inline unsigned long vlc_threadid (void)
      return v.i;
 }
 
-#if HAVE_EXECINFO_H
+#if defined(HAVE_EXECINFO_H) && defined(HAVE_BACKTRACE)
 # include <execinfo.h>
 #endif