[coq] Add "-q" to default flags.

Fixes ocaml#3924

Actually this PR highlights a problem in the way we handle flags from
profile and from the `theory` stanza, they should form a single set I
think, however they are disjoint.

What should be done here?

CHANGES.md

@@ -69,6 +69,8 @@ Unreleased
 - `dune describe` now also includes information about executables in addition to
   that of libraries. (#3892, #3895, @nojb)
 
+- [coq] Add `-q` flag to default `coqc` flags , fixes #3924, (# , @ejgallego)
+
 2.7.1 (2/09/2020)
 -----------------
 

src/dune_rules/coq_rules.ml

@@ -101,14 +101,16 @@ module Context = struct
     let dir = Path.build (snd t.coqc) in
     Command.run ~dir ?stdout_to (fst t.coqc) args
 
+  let standard_coq_flags = Build.return ["-q"]
+
   let coq_flags t =
     Build.(
       map ~f:List.concat
         (all
            [ Expander.expand_and_eval_set t.expander t.profile_flags
                ~standard:(Build.return [])
            ; Expander.expand_and_eval_set t.expander t.buildable.flags
-               ~standard:(Build.return [])
+               ~standard:standard_coq_flags
            ]))
 
   let theories_flags =