ocamlbrew requires m4

[hbbio]

ocamlbrew fails with the following error in /tmp/ocamlbrew.RANDOMID:

=-=- ocamlfind.1.5.5 troubleshooting =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
=> Could not build ocamlfind. The most common reason for that is a missing 'm4'
   system package.

Best would be to check at the beginning of the script if m4 is present.

[hcarty]

Good point. If ocamlfind support is going to stick around directly in ocamlbrew it's worth adding a check for prerequisites.