Reported by David Edelsohn <dje.gcc@gmail.com> in
<https://gcc.gnu.org/pipermail/gcc-patches/2023-November/636830.html>,
<https://gcc.gnu.org/pipermail/gcc-patches/2023-November/636843.html>.
* m4/threadlib.m4 (gl_THREADLIB_BODY): If --disable-threads was
specified, define AVOID_ANY_THREADS to 1.
* lib/mbtowc-lock.c: Omit all code if --disable-threads was specified.
* lib/nl_langinfo-lock.c: Likewise.
* lib/setlocale-lock.c: Likewise.
* lib/mbtowc-lock.h: Omit locking code if --disable-threads was
specified.
* lib/nl_langinfo.c: Likewise.
* lib/setlocale_null.c: Likewise.
* lib/mbrtowc.c: Don't include any multithreading headers if
--disable-threads was specified.
* lib/mbrtoc32.c: Likewise.