libctf doesn't build with Solaris sed

[Nick Alcock]

On 21 Oct 2020, Tom Tromey told this:

> Nick> We could, yes, but the whole point of this dance is that ctf-errors.h is
> Nick> generated from ctf-api.h, so that new errors and their descriptions only
> Nick> need to be added in one place (the API definition itself) and then
> Nick> everything else is generated from there. The only way we could use a
> Nick> .defs file is to generate ctf-api.h itself, and... that feels likely to
> Nick> annoy people who are using the copy in the source tree for reference
> Nick> purposes.
>
> No, with this approach no separate build steps are needed, and there's
> still only a single place to edit.
>
> See the appended.

You have my enthusiastic ack for this! Simpler and easier to understand
and one less sed script from hell.

(I've added more errors in my local tree, but feel free to just push if
it builds on Solaris now (which it probably does since the hell-sed is
gone): I'll adapt.)