interface setbuffer(), and emulates setbuffer() on USG systems using a
#define of setbuffer() in terms of setvbuf(). The #define is correctly
ifdefed in some places but was not correctly ifdefed here -- i.e., BSD
was essentially configured as USG here. This became fatal when <stdio.h>
was de-__P(())ified without testing. This file gets included before
<stdio.h>, so the #define now affects (and breaks)
`setbuffer<left parentheses>' in <stdio.h> where it didn't affect
`setbuffer<whitespace>'.
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