* guix/packages.scm (package-transitive-target-inputs,
  package-transitive-native-inputs): New procedures.
  (package-derivation): Parametrize `%current-target-system'.
  (package-cross-derivation): Implement.
* guix/utils.scm (%current-target-system): New variable.
* tests/packages.scm ("package-cross-derivation"): New test.
* doc/guix.texi (Defining Packages): Document
  `package-cross-derivation'.