DESTDIR. This avoids redundant information in the path when DOCDIR
points to some directory that already implies (or specifies) a certain
language. This is the case with the web site, where the release notes
are already installed under a language-specific directory. This
behavior is not being made mandatory because it might still be useful
to install all the translations in one directory, such as during
testing, or in a hypothetical release notes archive. Furthermore, it
is not being made the default because that breaks consistency with
stuff under doc/.
Reviewed by: bmah