It turns out that as part of POSIXification we got rid of checks for

wcwidth and mbtowc (because they're part of POSIX) but we didn't turn on
MULTIBYTE_SUPPORT because configure was testing to see if those functions
were supported ... and because the tests never ran, it always came back
as not being supported.  Time to bite the bullet and turn MULTIBYTE_SUPPORT
on by default.