cpp problem found by imake

[Dave Brolley]

Zack Weinberg wrote:

> I don't want to give the impression that we'll definitely break imake.
> My main concern is that supporting imake clashes with standards
> conformance.  You see, the result of pasting '<' with 'X11' is not a
> legal preprocessor token.  This is explicitly undefined behavior, and
> I'd like to issue warnings or reject.

There's nothing that says that our choice of undefined behaviour
can't be something reasonable. In this case a reasonable
'undefined' behaviour could be to do what imake expects.

Dave