functions: warn if the interpreter is missing when adding scripts with add_binary

Executables that are actually only scripts require the binary that is specified in the shebang to be able run.

The add_binary function will only warn and not automatically include the required interpreter. This is because with things like #!/bin/sh, it may be better to use the base hook that provides it instead of pulling in the bash binary. Also it is possible that the interpreter is simply added after the binary that requires it. That is why it says "Possibly missing".

Fixes #136 (closed).