Avoid arithmetic overflow when size is near max.
Depend on _POSIX_SOURCE and BSIZE, not _POSIX_VERSION, for
compatibility with system.h.
(BSIZE): Remove definition, since if BSIZE is not defined
we're never invoked.
of unused variable so that after preprocessing, the file will never
be empty. Otherwise, some compilers in ANSI mode (among them, the
Dec Alpha OSF 2.0 C compiler) reject empty files. From Kaveh Ghazi.
