lib/unicore/mktables needs to get File::Spec from ext/Cwd/lib

lib/unicore/mktables needs to get File::Spec from ext/Cwd/lib

It was only parallel make order roulette that it was working on my machine.
VMS doesn't have this problem, as it's invoking mktables from the top level,
using its -C option to change directory, and VMS's $(MINIPERL) has an -I option
for [.ext.Cwd.lib]

Maybe all platforms should swap to using the -C approach.