tools/ocaml: ask compiler for correct library

tools/ocaml: ask compiler for correct library

OCaml libraries will live in /usr/local/ if the user compiles OCaml
from source. This patch asks the OCaml compiler where we should look
for libraries.

NB: it may be that we should do the same thing for the NetBSD case,
but I don't have a BSD box to test this out.

Signed-off-by: Mike McClurg <mike.mcclurg@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>