[lmi] PATCH: Fix build with gcc11 in C++20 mode

[Vadim Zeitlin]

 Hello,

 The promised changes fixing the build with gcc11, now used by Debian Sid
and hence for the CI builds, are at

        https://github.com/let-me-illustrate/lmi/pull/193

or xanadu/gcc11-fixes branch, if you prefer.

 I'm not going to describe them in details here, but I'd just like to say
that even though there are 6 commits, most of them are pretty trivial and
the remaining ones are quite small too, so it shouldn't take much of your
time or energy to review them.

 Of course, it's possible that you disagree with my approach which, as was
outlined in the original message, is to fix a couple of warnings that were
simple to fix and disable the remaining -Wdeprecated-enum-float-conversion
one. But as I don't know in which direction you might disagree, i.e. if you
would prefer to disable the other warnings too, or if, on the contrary, it
could be better to fix, rather than disable, the remaining warning, I am
submitting this as is and will wait for your opinion about it.

 But if you don't disagree too much, it would be nice to merge this in
order to fix the CI builds.

 Which, BTW, does bring up another question: do we really want to use Sid
for them? We've started doing it because only Sid had the (cross) compiler
recent enough for our needs, but we could switch to using the new stable
Debian release (Bullseye) which has the same version of it as Sid right
now (but still uses, and will keep using, gcc 10.2). Personally I think
that we ought to switch to Bullseye as the official build platform, and use
it for the CI builds too, but we could still keep running the CI builds
under Sid to learn in advance of any new breakages due to compiler updates.

 What do you think?
VZ

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