t/rake-conditional-compilation

Allow building some targets only, eg "neko".
If the user tries to use a platform he didn't compile he'll get a nice message

Conditional compilation requires cppo (which is much faster and simpler than camlp4)

RAKE
====
implement a new build system based on rake.
The Rakefile documents library dependencies.

TODO:
- make ./*.ml files depend on libs/* directly rather than assuming the user
  compiles libs first
- don't hardcode libraries