ocamldoc Makefile: use .opt versions of the build tools when available
ocamldoc Makefile: use .opt versions of the build tools when available

On my machine, a sequential build of (make all allopt) goes from 31s
to 11s, and a parallel build (-j5) goes from 16s to 6s.

gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml,gerdstolpmann/ocaml