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 - When the encoding can't encode a definition it transform it into an
   axiom. But it's not the same than before. The differences is that
   eliminate algebraic is before this transformation into axiom and
   not after. So if the definition start with a match, in one case
   that give different nice axioms, and in the other case that
   introduce a match function which is polymorph and not well used by
   provers.

   Possible solutions :
   1) make the match function easier to use with some modification to
   its definition and discriminate it.
   2) Eliminate the match function when the encoding tranform a
   definition into axioms.

This reverts commit 1f652980.

Moreover,
- Coq realizations are disabled for Coq < 8.3 due to coqdep bugs,
- Whyconf.magicnumber is incremented to ensure coqc and coqide are called
  with -R,
- realizations for FP arithmetic are disabled if Flocq is missing.

Note that this is just a consequence of symbols not being imported from
realizations. If Coq files were generated with "Require Import" directives
rather than just "Require", the change would have been completely
transparent. This is not reason enough to switch to "Require Import" but
it should be kept in mind.

My first idea was to use "-R @libdir@/coq", or some other variable I would
have defined in configure.in, but it didn't work at all. Indeed, such path
variables depend on cascaded substitution, which work fine inside
make files and shell scripts, but not at all inside Why3 config files.
Note that this is a documented feature of Autoconf so I doubt there is any
way to circumvent it.

So I ended up adding a new format specifier inside call_provers: %l is
substituted by Config.libdir.