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.
This file doesn't need to be created initially, as it will be done
automatically during the build, but it does need to be updated whenever
setup0.h changes, so explain this in both the README-GIT.md file and the
main build instructions.
Closes#18699.
Rename BuildGit.txt to README-GIT.md, which is a more standard name, and
a better format for viewing this file online, e.g. from GitHub Web UI,
and link it from the main README to make it easier to find.
