* libc/include/stdio.h (__SNLK): Define.

* libc/include/stdio.h (__SNLK): Define.
* libc/include/stdio_ext.h (FSETLOCKING_QUERY, FSETLOCKING_INTERNAL,
FSETLOCKING_BYCALLER): Define.
(__fsetlocking): Declare.
* libc/stdio/Makefile.am: Build fsetlocking.c.
* libc/stdio/Makefile.in: Regenerate.
* libc/stdio/fsetlocking.c: New file.
* libc/stdio/local.h (_newlib_flockfile_start): Make _flockfile
call dependent on __SNLK flag.
(_newlib_flockfile_exit, _newlib_flockfile_end): Ditto for
_funlockfile calls.
Define all locking macros as empty if __SINGLE_THREAD__.
* libc/stdio/stdio.tex: Include fsetlocking.def.