Fix minor alloc.c problems found by static checking.

Fix minor alloc.c problems found by static checking.

* alloc.c (_malloc_internal, _free_internal) [!DOUG_LEA_MALLOC]:
New extern decls, to avoid calling undeclared functions.
(dont_register_blocks): Define if ((!SYSTEM_MALLOC && !SYNC_INPUT)
&& GC_MALLOC_CHECK), not if ((GC_MARK_STACK || defined
GC_MALLOC_CHECK) && GC_MALLOC_CHECK), to match when it's used.
(NEED_MEM_INSERT): New macro.
(mem_insert, mem_insert_fixup) [!NEED_MEM_INSERT]: Remove; unused.