pre-GC_set_start_callback compatibility

pre-GC_set_start_callback compatibility

* configure.ac: Add a check for GC_set_start_callback.

* libguile/gc.c (scm_i_gc): If we don't have GC_set_start_callback, run
  the before-gc hook manually here.
  (scm_init_gc): Otherwise set it as a start callback.

* libguile/hashtab.c (weak_gc_callback, weak_gc_hook)
  (weak_gc_finalizer, scm_c_register_weak_gc_callback): Fix to work
  either way, with or without GC_set_start_callback.