Make sure that we have a real on-the-stack alloca()

Make sure that we have a real on-the-stack alloca()

Because of how Guile saves and restores continuations (by copying the
stack), and how it uses alloca to create space for debug information
on the stack, we must have an alloca() that really does use the stack,
and not one that uses the heap.

To do this, we use the Gnulib "alloca-opt" module instead of "alloca".

This commit also updates the Gnulib sources from the current Gnulib
Git repository.