BUILD/MEDIUM: rebuild everything when an include file is changed

BUILD/MEDIUM: rebuild everything when an include file is changed

Some users tend to get caught by incorrect builds when they try patches
that modify some include file after they forget to run "make clean".
While we can't blame users who are not developers, forcing developers
to rely on a painful autodepend is not nice either and will cause them
to test their changes less often. Here we propose a reasonable tradeoff.
This patch introduces a new "INCLUDES" variable which enumerates all
the ".h" files and sets them as a build dependency for all ".o" files.
This list is then copied into a "DEP" variable which can safely be
overridden if desired. This way by default all .c files are rebuilt if
any include file changes. This is the safe method for all users. And
developers can simply add "DEP=" to their quick build scripts to keep
the old fast and efficient behaviour.