Don't build locale/wcwidth junk if NLS is disabled.

These functions are only used for NLS support, so it is silly to
compile them if NLS is disabled for any reason.

To do this, we create a local "dx-nls" gnulib module which can in
turn use conditional dependencies to pull in all the (now optional)
modules.

It is necessary to update gnulib in order for this to actually work,
as otherwise uniwidth/width is still compiled for some reason, and
it looks like this has been fixed upstream.