cpplib: fix -Wtrigraphs double warnings

[Neil Booth]

Zack Weinberg wrote:-

> 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.

Yes, there was something weird with dejagnu.  I had to add the flag
-fno-show-column to get the 2nd line to work in that testcase I just
posted.

I've had this before: random tests don't work like they should without
adding -fno-show-column.  It's probably a regexp problem in dejagnu
somewhere.  Shrug.

Neil.