Pure makefiles

This branch is an attempt to get rid of ocamlbuild and, instead, use plain Makefiles for compiling the OCaml files and produce the final executables.

Advantages:
+ Better parallelism (make -j)
+ Hopefully easier to port to Windows.

Drawbacks:
- make is based on timestamps while ocamlbuild is based on file contents.  Hence, after every "make extraction", all extracted OCaml files need recompiling.  Before, with ocamlbuild, only those extracted files whose contents changed were recompiled.