Simplify things by putting setup.h files themselves under version
control and getting rid of setup0.h ones.
The initial motivation for using separate setup0.h files was to allow
having local changes to setup.h, but with Git there is a simple way to
do it by using "git update-index --skip-worktree include/wx/msw/setup.h"
for example, so we don't really need setup0.h any more and dropping them
makes things simpler.
Use "git submodule foreach" to run "git archive" in all of the
submodules as well.
Also document the requirement to have GNU tar which is needed to be able
to successfully extract several concatenated tar archives.
