Support `make cold` to build OPAM without an OCaml compiler being available.

This is useful on e.g CentOS 6.4 which only has OCaml 3.11.2 packaged.
Bootstrap: build using native code
bootstrap: typo in Makefile
Merge pull request #910 from avsm/cold-bootstrap

Support `make cold` to build OPAM without an OCaml compiler being available