* configure.in: New check for uca lib (needed for IA64 on HP-UX).

* configure.in: New check for uca lib (needed for IA64 on HP-UX).

* threads.c (SCM_MARK_BACKING_STORE): Use scm_ia64_ar_bsp() and
scm_ia64_register_backing_store_base() instead of Linux-specific
implementations.

* gc.h (scm_ia64_register_backing_store_base, scm_ia64_ar_bsp):
New declarations.

* gc.c (__libc_ia64_register_backing_store_base): Declaration
removed.
(scm_ia64_register_backing_store_base, scm_ia64_ar_bsp): New, with
implementations for Linux and HP-UX.

* coop-pthreads.c (SCM_MARK_BACKING_STORE): Use scm_ia64_ar_bsp()
and scm_ia64_register_backing_store_base() instead of
Linux-specific implementations.

* continuations.h (__libc_ia64_register_backing_store_base):
Declaration removed.
(scm_t_contregs): New "fresh" field.

* continuations.c (ia64_getcontext): Removed.
(scm_make_continuation): Use continuation fresh field instead of
interpreting getcontext return values (which isn't portable).  Use
scm_ia64_ar_bsp() and scm_ia64_register_backing_store_base()
instead of Linux-specific implementations.
(copy_stack_and_call): Use scm_ia64_register_backing_store_base()
instead of Linux-specific implementation.

* _scm.h (__ia64__): Also detect __ia64.