[Rivet] auxiliary scripts in several generators.

[David Grellscheid]

Hi all,

the automake way to deal with a staging location is to use the DESTDIR
variable during the temporary installation, but set --prefix to the
final location. Your use case is the reason DESTDIR exists.

Pere's suggestion does not work, as it now hard-codes e.g. the relative
positions of /lib and /include.

  David