multiple inclusion protection.
(_POSIX_SYNCHRONIZED_IO, _POSIX_FSYNC, _POSIX_MAPPED_FILES,
_POSIX_MEMORY_PROTECTION, _POSIX_POLL, _POSIX_SELECT): New macros,
define all these to 1.
(_POSIX_SYNC_IO): Add #undef, to remind us that pathconf reports it.
1998-08-12 Roland McGrath <roland@baalperazim.frob.com>