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.