run finalizers asynchronously in asyncs

run finalizers asynchronously in asyncs

* libguile/finalizers.c: New excitement!  We'll be running finalizers
  asynchronously, from asyncs.  This will make it safer to allocate
  while holding a mutex.
  (GC_set_finalizer_notifier): Add back-compat shim.

* libguile/init.c (scm_i_init_guile): Init the async finalizer mechanism
  during boot.

* libguile/gc.c (scm_storage_prehistory): Tell libgc we'll be finalizing
  on demand.
  (scm_gc): Explicitly run finalizers here.

* libguile/threads.c (guilify_self_2): Run finalizers here if
  queue_finalizer_async happened to run during guilify_self_1.

* configure.ac: Add check for GC_set_finalizer_notifier.