Move declaration of sbrk. Maybe include gc.h. Make

Move declaration of sbrk.  Maybe include gc.h.  Make
marking/sweeping code dependent on BOEHM_GC.
(EXCEEDS_LISP_PTR): New.
(MAX_SAVE_STACK) [BOEHM_GC]: Define as 0.
(Vmost_positive_fixnum, Vbuffer_alist, initialized)
(GC_print_stats) [BOEHM_GC]: Declare.
(staticvec, inhibit_gc_count) [BOEHM_GC]: Don't declare.
(xcalloc): New.
(gc_out_of_memory, xgc_malloc, xgc_realloc, xgc_free)
(finalize_buffer_marker_chains, custom_finalize) [BOEHM_GC]: New.
(lisp_malloc, make_interval, allocate_string)
(allocate_string_data, make_float, Fcons, allocate_vectorlike)
(Fmake_symbol, allocate_misc, survives_gc_p, init_alloc_once)
(init_alloc):  Add BOEHM_GC cases.
(lisp_free, pure_alloc, inhibit_garbage_collection) [BOEHM_GC]:
Don't define.
(float_block, float_block_index, n_float_blocks, float_free_list)
(init_float, cons_block, cons_block_index, cons_free_list)
(n_cons_blocks, init_cons, all_vectors, n_vectors, symbol_block)
(symbol_block_index, symbol_free_list, n_symbol_blocks)
(init_symbol, marker_block, marker_block_index, marker_free_list)
(n_marker_blocks, init_marker):  Declare static.
(fmake_marker): Use XSET_MARKER_CHAIN.  Add BOEHM_GC case.
(mark_maybe_object, mark_maybe_pointer): Use MARKER_CHAIN.
(inhibit_garbage_collection): Use Vmost_positive_fixnum.
(extra_gc_work): New, extracted from Fgarbage_collect.
(Fgarbage_collect): Add BOEHM_GC case.  Use MARKER_CHAIN.