Extensions should check for the lowest version of dune they are available in #2957
Description
Looking at some failure logs, I came across this one:
#=== ERROR while compiling ppxlib.0.10.0 ======================================#
# context 2.0.5 | linux/x86_64 | ocaml-base-compiler.4.04.2 | file:///home/opam/opam-repository
# path ~/.opam/4.04.2/.opam-switch/build/ppxlib.0.10.0
# command ~/.opam/4.04.2/bin/dune build -p ppxlib -j 71
# exit-code 1
# env-file ~/.opam/log/ppxlib-6-f9b82b.env
# output-file ~/.opam/log/ppxlib-6-f9b82b.out
### output ###
# File "dune-project", line 4, characters 7-13:
# 4 | (using cinaps 1.0)
# ^^^^^^
# Error: Unknown extension "cinaps".
The problem is that dune 1.6 is installed but cinaps is only available in dune 1.11 and above.
However the dune-project file is as follow:
(lang dune 1.0)
(name ppxlib)
(using cinaps 1.0)
In my opinion dune should be able to raise an error if such dune-project gets detected. In that case an error should be raised if (lang dune ...) is below 1.11.