GNUmakefile: simplify recursive logic and eliminate a "pdflatex" call.

In the recursive $(MAKE), the sub-$(MAKE) can succeed and then
subsequently delete its "previous" file. When that returns control to
the parent $(MAKE), the parent will try to diff the (now missing)
"previous" file against the latest one, which of course fails. This
leads to an extra build of the same, current, not-gonna-change PDF.

A slight change in the logic both simplifies things and eliminates
that extra rebuild. Thanks to Wojtek Kosior who pointed out the bug in
an email.