Add --strict to menhir flags

[nomeata]

so that
#310 (comment)
does not happen by accident again.

When developing, you can make it less strict, by passing

make MENHIR_FLAGS="--infer --dump --explain"

similar to how you can make ocaml less pedantic by passing, say,

make OCAML_FLAGS="-cflags '-w +a-4-27-30-42-44-45 -g'"

(which I usually do).