build-system/gnu: `package-with-explicit-inputs' skips non-GBS packages.

* guix/build-system/gnu.scm (package-with-explicit-inputs)[rewritten-input]:
  Leave P unchanged if its build system is not GNU-BUILD-SYSTEM.

guix/build-system/gnu.scm

@@ -72,7 +72,11 @@ (define rewritten-input
       (memoize
        (match-lambda
         ((name (? package? p) sub-drv ...)
-         (cons* name (loop p) sub-drv))
+         ;; XXX: Check whether P's build system knows #:implicit-inputs, for
+         ;; things like `cross-pkg-config'.
+         (if (eq? (package-build-system p) gnu-build-system)
+             (cons* name (loop p) sub-drv)
+             (cons* name p sub-drv)))
         (x x))))
 
     (package (inherit p)