Cannot use --enable-clocale=darwin

[Jonathan Wakely]

It's just been pointed out to me that configuring with
--enable-clocale=darwin fails:

checking for underlying I/O to use... stdio
configure: error: Unknown argument to enable/disable clocale

because it's not listed in the valid options:

AC_DEFUN([GLIBCXX_ENABLE_CLOCALE], [
  GLIBCXX_ENABLE(clocale,auto,[[[=MODEL]]],
    [use MODEL for target locale package],
    [permit generic|gnu|ieee_1003.1-2001|newlib|yes|no|auto])

I assume there's no good reason for that and it could be added?