Lazy ocamlfind in makefile

Do not merge. This is a (useless) proof of concept only.

Having been running in circles the whole day between #93, #88 and what I suggested in #92, it seems in fact hard to boil down to per package ocamlfind calls (which I guess @samoht realized long time ago). But I fell on that [hack](http://lists.gnu.org/archive/html/automake-ng/2012-05/msg00077.html) to memoize values in makefiles. I just tried a proof of concept implementation which would optimize calls to ocamlfind per ordered set of packages. But on my examples it doesn't seem to make any difference since what we had in e767148.