Cannot use --enable-clocale=darwin

[Benjamin De Kosnik]

> 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?

Yes, it should be added. Note, I don't know if configuring this way
works, clearly it's not been tested...

-benjamin