Rename GNULIB_WCHAR_SINGLE to GNULIB_WCHAR_SINGLE_LOCALE. * lib/lc-charset-dispatch.c: Test GNULIB_WCHAR_SINGLE_LOCALE instead of GNULIB_WCHAR_SINGLE. * lib/wcwidth.c: Likewise. * tests/test-wcwidth.c: Likewise. * doc/multithread.texi: Document GNULIB_WCHAR_SINGLE_LOCALE instead of GNULIB_WCHAR_SINGLE. * NEWS: Document the change.