Add :when{-system,-executable}? field to recipe

[Pitometsu]

It's a feature request. Is it hard to implement another few fields for recipes like :when-system and :when-executable? And implement it like this:

(when (executable-find "ghc") ...)

(when (eq system-type 'darwin) ...)

This let install some packages only for target OS and only if required executables found.

[npostavs]

I think it would make more sense to have a single :when field where you can put whatever lisp expression you want.