* alloc.c: Handle unusual debugging option combinations.

* alloc.c: Handle unusual debugging option combinations.

(GC_CHECK_MARKED_OBJECTS): Undef if ! GC_MARK_STACK,
since the two debugging options are incompatible.
(GC_MALLOC_CHECK): Similarly, undef if GC_CHECK_MARKED_OBJECTS
is defined.
(mem_init, mem_insert, mem_insert_fixup):
Define if GC_MARK_STACK || GC_MALLOC_CHECK.
(NEED_MEM_INSERT): Remove; no longer needed.