Currently the encoding of documents generated by DocBook depends on
the current locale.  Make the output reproducible independently of
the locale, by setting the encoding to UTF-8 (LC_CTYPE=C.UTF-8) by
preference, or ASCII (LC_CTYPE=C) as a fallback.

LC_CTYPE can normally be overridden by LC_ALL, but the top-level
Makefile unsets that.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
[jc: added check-lc_ctype to .gitignore]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>