Use single-colon rules in makefiles

Use single-colon rules in makefiles

Double-colon rules allow the flexibility to specify commands in
multiple places, but they also make the order of commands and
dependencies dependent on the order of declarations in the Makefile.
Convert all of our double-colon rules to single-colon rules, with the
exception of "clean-unix" and "clean-windows" which have commands
defined in both post.in and in numerous Makefile.in files.