Teaching regex operators about collations

Since ILIKE now responds to collations, it would be nice if the
case-insensitive regex operators did too.  The hard part of that is
getting the information from src/backend/utils/adt/regexp.c to
src/backend/regex/regc_locale.c.  In principle we could probably add
a field to the data structures carried around in the regex library,
but that is looking a bit invasive, and since we share that code with
the Tcl project I'm loath to change it too much.  So what I'm thinking
about is just having a couple of static variables in regc_locale.c that
we initialize before each use of the regex library.  This is a bit
grotty, but there's no need for the regex library to be re-entrant,
so it wouldn't cause any problems until that improbable day when
somebody succeeds in multi-threading the backend.

Comments?
        regards, tom lane