Restore the `make cold` target that builds without a system compiler.

This was removed as part of the Makefile refactoring when switching
from `ocp-build` to `make`.  We now use OCaml 4.02beta1 to bootstrap
since it's significantly faster than 4.01 (no camlp4) and works without
patches on MacOS X (the `-fno-defer-pop` issue).