lo: use --with-export-validation

First I was scared that that java monster this brings in slows down the
toplevel noop 'make', but in fact it doesn't really do that:

--without-export-validation:
real    6m4.528s
user    27m15.995s
sys     0m17.497s

--with-export-validation:
real    7m4.533s
user    29m42.029s
sys     0m20.959s

Probably the bottleneck is mostly still the Writer tests while the
validation is not enabled there yet.