why3-base is not compatible with ocaml 4.07

[kit-ty-kate]

cc @silene

Build log:

#=== ERROR while compiling why3-base.0.88.3 ===================================#
# context              2.0.0~rc | linux/x86_64 | ocaml-variants.4.07.0+trunk | git+file:///home/opam/opam-repository
# path                 ~/.opam/4.07.0+trunk/.opam-switch/build/why3-base.0.88.3
# command              /usr/bin/make -j71 all opt byte
# exit-code            2
# env-file             ~/.opam/log/why3-base-1-58a313.env
# output-file          ~/.opam/log/why3-base-1-58a313.out
### output ###
# [...]
# Ocamlc   lib/ocaml/why3__IntAux.ml
# Ocamlc   src/util/number.ml
# Ocamlc   src/util/loc.ml
# Ocamlc   src/util/debug.ml
# File "compiler internals", line 1:
# Error: Unbound module Stdlib
# Ocamlc   src/session/xml.ml
# Ocamlc   src/util/plugin.mli
# Makefile:1881: recipe for target 'src/util/stdlib.cmi' failed
# make: *** [src/util/stdlib.cmi] Error 2
# make: *** Waiting for unfinished jobs....
# Ocamlopt src/util/cmdline.ml

[camelus]

✅ All lint checks passed eb4db24

• These packages passed lint tests: why3-base.0.88.1, why3-base.0.88.2, why3-base.0.88.3

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✅ Installability check (8700 → 8700)