Because plpgsql's scanner uses %option case-insensitive, flex's results could

Because plpgsql's scanner uses %option case-insensitive, flex's results could
theoretically vary depending on what the compile-time locale setting is.
Hence, force it to see LC_CTYPE=C to ensure consistent build results.
(It's likely that this makes no difference in practice, since our
specification for "identifier" surely includes both ends of any possible
uppercase/lowercase pair anyway.  But it should silence warnings about
ambiguous character classes that are reported by some buildfarm members.)