(add-after 'unpack 'fix-cross-compile
  ;; we ignore all the arguments by simply binding them to _
  (lambda _
    ;; substitute* is a utility macro provided by guix that allows
    ;; us to make small patches to files
    (substitute* "Makefile"
      ;; ungexp #$... allows us to access values outside the build
      ;; environment within a gexp
      (("pkg-config")
       #$(pkg-config-for-target)))))