More fixes for situations where ocamlopt is not available

- run bindtests.opt only if available
- use ocamlc -custom
- Don't try to install native code if it hasn't been built (Thanks to Olaf Hering)
Fix out-of-tree appliance build
Fixes for various tests in out-of-tree builds
Fix out-of-tree build: Do not accidentally build mllib components in srcdir