cpplib: fix -Wtrigraphs double warnings

[Zack Weinberg]

On Wed, Nov 22, 2000 at 08:22:28PM +0000, Neil Booth wrote:
> And a testcase (Wtrigraphs.c):-

I'm not sure this will fail in the presence of the bug.  The existing
trigraphs test should've picked it up, but it didn't.

I don't know what to do about it, though; it's a design problem with
dg.exp.  The "excess errors" test doesn't distinguish between one copy
of an error mesage, and multiple copies.

zw