When ocaml is installed in an unusual location, help cc find the headers

The problem is triggered by using an OCaml compiler installed from OPAM
where <caml/mlvalues.h> needs to be looked up relative to something like

  $HOME/.opam/4.00.1/lib/ocaml
make install now understands DESTDIR

This helps make binary packaging (e.g. RPM, deb, ...) easier.
Assume DESTDIR points to the build chroot itself ...

... and not the ocaml library directory within the chroot.

If I disable ocamlfind then I get this:

$ make install DESTDIR=/tmp/without-ocamlfind
install -d /tmp/without-ocamlfind/usr/lib64/ocaml/oclock
install -t /tmp/without-ocamlfind/usr/lib64/ocaml/oclock oclock.cma ...

If I enable ocamlfind then I get this:

$ make install DESTDIR=/tmp/with-ocamlfind
ocamlfind install oclock oclock.cma oclock.cmxa liboclock.a ...
Installed /tmp/with-ocamlfind/usr/lib64/ocaml/oclock/oclock.a
...
Merge pull request #2 from djs55/fix-opam-build

Fix build under OPAM; make binary packaging easier