GNUmakefile,examples: overhaul

GNUmakefile,examples: overhaul

First, rename GNUmakefile to GNUmakefile.mjo. This contains most of
the logic and has been updated to leave MJOTEX, BIBS, INDICES, and so
on undefined. It is not intended to be invoked directly.

The entire "examples" document has been moved to an "examples"
directory with its own GNUmakefile that declares a few variables and
then includes GNUmakefile.mjo (via kpsewhich). The idea here is that
new projects should be able use a minimal GNUmakefile, offloading most
of the work to GNUmakefile.mjo, wherever it lives.