FYI: C++ & --prefix

[Akim Demaille]

While I was on the documentation, I noticed some uses of
@ifinfo that seemed wrong to me.  There remains one that
we should carefully check: I'm afraid that @ifinfo + @iftex,
as is the case currently, does not cover all the cases
(i.e., I doubt @ifinfo is activated for html for instance).

This patch reveals a limitation of the current glr.cc:
yyUserAction, which executes the user actions, is not
in the namespace, hence, even to use a yy:: feature,
the user needs to make the yy:: part explicit.  Maybe
issuing a "using namespace yy" with yyUserAction would
do the trick.  But I dislike "using namespace".

diffs.patch
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