Print a C backtrace in ecl_thread_internal_error

After the recent changes to the C backtrace interface,
    _ecl_dump_c_backtrace() no longer needs a working lisp
    environment, so we can safely call it here.

src/c/error.d

@@ -71,6 +71,7 @@ ecl_thread_internal_error(const char *s)
     fprintf(stderr, "  [%d: %s]\n", saved_errno,
             strerror(saved_errno));
   }
+  _ecl_dump_c_backtrace();
   fprintf(stderr,
           "\nDid you forget to call `ecl_import_current_thread'?\n"
           "Exitting thread.\n");