ports/186046: x11/dgs : fix build with current texinfo

[Christoph Moench-Tegeder]

## gerald at FreeBSD.org (gerald at FreeBSD.org):

> Christoph, one question:  I could not reproduce the failure in any
> of my test builds.  Am I right that this won't happen for "isolated"
> builds a la tinderbox, but for users building ports on a "regular"
> system with more than the minimally necessary set of packages installed?
> And specifically here a newer version of texinfo?

Exactly.
I came across then when installing math/octave - octave depends on
print/texinfo and graphics/GraphicsMagick13, which in turn depends on
x11/dgs.
As I've seen a couple of such texinfo problems (texinfo upstream tightened
the rules on "acceptable texi files" in version 5.0), I'm thinking about
a general fix for this by having TEXI2HTML etc. defined to the "right"
version of texinfo, but that's just an idea...

Rergards,
Christoph

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