Configure: unbreak -S option now that -O is the default

Configure: unbreak -S option now that -O is the default

As far as I can tell, using the -S and -O options together has always
yielded an error of this form:

    Configure: 2042: .: Can't open ./optdef.sh

That's because, even though optdef.sh is created in the UU directory, and
most of Configure is run in that directory, part of the -S implementation is
run in the root directory, and was therefore trying to read ./optdef.sh
instead of ./UU/optdef.sh.

As of 41d73075f0801c26794dadb1ff690f305d7e53a7, the -O mode is always
enabled, so the -S option has been broken since then. This fixes that.