Makefile: make all calls to ocamlfind indirect.

Fixes #89. Drastically improves build peformance in some cases since
we call ocamlfind less (from 17s to 10s on a large example that uses
~15 packages, from 2.9s to 2.5s for assemblage).
Merge pull request #92 from dbuenzli/fix-89

Fix #89, no more direct calls to ocamlfind.