GC tweaks

[Henry Cejtin]

The gc code is not type correct: the function dfsPop() in GC_size.c returns a
pointer and takes a single GC_state argument.  It is  passed  to  maybeCall()
and  GC_foreachPointerInObject(),  which requires it to take 2 arguments, the
second one being a (pointer *) and to return void.

This could actually cause bad code, although it is unlikley.   I  don't  know
why  GCC  didn't detect this, but my guess is that it doesn't check the types
on function pointers.