TT>STACKBOTTOM</tt>.
<TT>STACK_GRAN</tt> should generally also be undefined and defined.
The cold end of the stack is determined by taking an address inside
<TT>GC_init's frame</tt>, and rounding it up to
the next multiple of <TT>STACK_GRAN</tt>.  This works well if the stack base is
always aligned to a large power of two.
(<TT>STACK_GRAN</tt> is predefined to 0x1000000, which is
rarely optimal.)
<DT><TT>HEURISTIC2</tt>
<DD>
May be defined instead of <TT>STACKBOTTOM</tt>.
The cold end of the stack is determ