another cpplib bug

[Zack Weinberg]

The appended file is mis-processed by cpplib: an `@' replaces the
first character of `init' on the line referring to ONCE.  If PREFIX is
defined to something other than the empty string, the bug does not
occur.

Please add this to the testsuite.  Patch for the bug will follow.

zw

-- cpp-bug-3.c --
#define P(a,b) P1(a,b)
#define P1(a,b) a##b

#define ONCE(x, y) (x ?: x = y())
#define PREFIX

extern int P(PREFIX, init) (void);

int
fun(void)
{
  static int memo;
  return ONCE(memo, P(PREFIX, init));
}