pgsql: Because plpgsql's scanner uses %option case-insensitive, flex's

Log Message:
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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.)

Modified Files:
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pgsql/src/pl/plpgsql/src:
Makefile (r1.30 -> r1.31)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/Makefile.diff?r1=1.30&r2=1.31)