To update tmux to a new version:

- Build the package from pkgsrc and write down all -D flags passed to the
  compiler.  Autoconf is not generating a config.h file, so this is the
  best we can do to get the build-time settings in place.
- Use prepare-import.sh to regenerate the dist directory.
- Update usr.bin/tmux/Makefile to sync the CPPFLAGS to the list of -D flags
  gathered earlier on.
- Update the list of source files in usr.bin/tmux/Makefile with the new
  dist/*.c listing.
- cvs import the contents of the new dist directory.
- Fix merge conflicts, if any.
- Commit the changes to the reachover Makefiles.
- Update doc/3RDPARTY with the new tmux version.
- Add a note to doc/CHANGES about the new version.