(purebeg, pure_size, pure_bytes_used_before_overflow):
New variables.
(init_alloc_once): Initialize new variables.
(PURE_POINTER_P): Use new variables.
(pure_alloc): If pure storage overflows, allocate from the heap.
(check_pure_size): New function.
(Fgarbage_collect): Don't GC if pure storage has overflowed.
(Vpost_gc_hook, Qpost_gc_hook): New variables.
(syms_of_alloc): DEFVAR_LISP post-gc-hook, initialize
Qpost_gc_hook.
(Fgarbage_collect): Run post-gc-hook.
(Fmake_symbol): Adapt to changes of struct Lisp_Symbol.