a no-op.
<DT><TT>CLEAR_DOUBLE</tt>
<DD>
If <TT>CLEAR_DOUBLE</tt> is defined, then
<TT>CLEAR_DOUBLE</tt>(x) is used as a fast way to
clear the two words at GC_malloc-aligned address x.  By default,
word stores of 0 are used instead.
<DT><TT>HEAP_START</tt>
<DD>
<TT>HEAP_START</tt> may be defined as the initial address hint for mmap-based
allocation.
<DT><TT>ALIGN_DOUBLE</tt>
<DD>
Should be defined if the architecture requires double-word alignment
of <TT>GC_malloc</tt>ed memory, e.g. 8-byte alignment with a
