Arrange so that `SCM_I_CURRENT_THREAD' is not accessed outside of libguile.

Arrange so that `SCM_I_CURRENT_THREAD' is not accessed outside of libguile.

* libguile/__scm.h (scm_async_tick): New declaration.
  (SCM_ASYNC_TICK)[!BUILDING_LIBGUILE]: Use `scm_async_tick ()'.

* libguile/async.c (scm_critical_section_start,
  scm_critical_section_end, scm_async_tick): New functions.

* libguile/async.h (scm_i_critical_section_mutex): Made internal.
  (scm_critical_section_start, scm_critical_section_end): New
  declarations.
  (SCM_CRITICAL_SECTION_START,
  SCM_CRITICAL_SECTION_END)[!BUILDING_LIBGUILE]: Use the same-named
  function (lower-case).

* libguile/stackchk.h (SCM_STACK_OVERFLOW_P): Conditionalize on
  `BUILDING_LIBGUILE'.

* libguile/threads.h (SCM_I_CURRENT_THREAD, scm_i_dynwinds,
  scm_i_set_dynwinds, scm_i_last_debug_frame,
  scm_i_set_last_debug_frame): Conditionalize on `BUILDING_LIBGUILE'.