Adjust to match changes in libgc's CVS (pre-7.2).

Adjust to match changes in libgc's CVS (pre-7.2).

In libgc CVS the `GC_do_blocking ()' declaration is now public but it
uses a slightly different signature for its first argument.

* configure.ac: Check for `GC_fn_type'.

* libguile/threads.c (GC_fn_type)[HAVE_GC_DO_BLOCKING &&
  !HAVE_GC_FN_TYPE]: New typedef.
  (scm_without_guile): Explicitly cast `without_guile_trampoline' to
  `GC_fn_type'.  This is because the upstream definition currently
  looks like `typedef void * (extern *GC_fn_type)(void *);', which
  isn't compatible.