autotools: Use non-recursive make

With the current testing infrastructure, we don't have a strictly
hierarchical set of dependencies. This causes problems with a recursive
make, and means we have to hack around some of the dependencies.

This change generates a single, top-level makefile from all of the
Makefile.am fragments. We still need the po/ directory as a separate
SUBDIR, but all others can be converted to non-recursive.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>