Remove warnings with OCaml 4.13

[kit-ty-kate]

In prevision of ocaml/ocaml#10207

[kit-ty-kate]

Now that ocaml/ocaml#10207 is merged this PR is good to go.

[dra27]

Please could you update the Makefiles as well?

num/src/Makefile

Line 21 in 5698bc4

CAMLCFLAGS=-w +a-4-9-41-42-44-45-48 -warn-error A -bin-annot -g \

and

num/toplevel/Makefile

Line 6 in 5698bc4

-w +a-4-9-41-42-44-45-48 -warn-error A \

[kit-ty-kate]

Done

[dra27]

There is a question over whether -warn-error A will definitely trigger the alert, but the change is benign as things stand. I'll merge: if num 1.4 still triggers the warning when 4.13 is branched then we'll know we need to release.