Make strfmon_l() work without requiring the use of <xlocale.h>.

The strfmon_l() function provided by <xlocale/_monetary.h> is also part
of POSIX 2008's <monetary.h>, so it should be exposed by default.

Change the check used in <monetary.h> to be similar to the one that's
part of <wchar.h>, where we both test for __POSIX_VISIBLE and
_XLOCALE_H_.