Avoid clash with system setjmp/longjmp on IA64

Problem was that if an application includes both libguile.h and the
system's setjmp.h, and is compiled on IA64, it gets compile errors
because of jmp_buf, setjmp and longjmp being multiply defined.

* libguile/__scm.h (__ia64__): Define scm_i_jmp_buf, SCM_I_SETJMP and
  SCM_I_LONGJMP instead of jmp_buf, setjmp and longjmp.

  (all other platforms): Map scm_i_jmp_buf, SCM_I_SETJMP and
  SCM_I_LONGJMP to jmp_buf, setjmp and longjmp.

* libguile/continuations.c (scm_make_continuation): Use `SCM_I_SETJMP'
  instead of `setjmp'.
  (copy_stack_and_call): Use `SCM_I_LONJMP' instead of `longjmp'.
  (scm_ia64_longjmp): Use type `scm_i_jmp_buf' instead of `jmp_buf'.

* libguile/continuations.h (scm_t_contregs): Use type `scm_i_jmp_buf'
  instead of `jmp_buf'.

* libguile/threads.c (suspend): Use `SCM_I_SETJMP' instead of
  `setjmp'.

* libguile/threads.h (scm_i_thread): Use type `scm_i_jmp_buf' instead
  of `jmp_buf'.

* libguile/throw.c (JBJMPBUF, make_jmpbuf, jmp_buf_and_retval): Use
  type `scm_i_jmp_buf' instead of `jmp_buf'.
  (scm_c_catch): Use `SCM_I_SETJMP' instead of `setjmp'.
  (scm_ithrow): Use `SCM_I_LONGJMP' instead of `longjmp'.

libguile/continuations.h

@@ -43,7 +43,7 @@ SCM_API scm_t_bits scm_tc16_continuation;
 typedef struct 
 {
   SCM throw_value;
-  jmp_buf jmpbuf;
+  scm_i_jmp_buf jmpbuf;
   SCM dynenv;
 #ifdef __ia64__
   void *backing_store;