Fix remaking of config.h and config.h.in

Fix remaking of config.h and config.h.in

Remaking config.h and config.h.in updates the target only when its
contents actually changes.  This is important, because after updating
config.h we need to recompile everything.

The make rules to do that are straight from the Autoconf manual.  But
they don't work: the rules that connect config.h and config.h.in to
stamp-h and stamp-h.in don't have recipes.  Since make doesn't have an
implicit rule either, it concludes that the target remains unchanged.
It still updates the prerequisites.  The recipe for updating the stamp
files then change the the targetes behind make's back.  Make misses
the change of config.h and/or config.h.in, and we get an incomplete
rebuild.

The rules need empty recipes instead.  This Autoconf manual was fixed
accordingly in autoconf.git commit 6b42b38.