Unlike https://issues.guix.gnu.org/42601, this happens _outside_ 
./pre-inst-env:

$ LC_ALL=C guix build mate-polkit
Backtrace:
In ice-9/boot-9.scm:
    222:29 19 (map1 (((gnu packages guile)) ((gnu packages image)) # ?))
    222:29 18 (map1 (((gnu packages image)) ((gnu packages #)) ((?)) ?))
    222:29 17 (map1 (((gnu packages kde-frameworks)) ((gnu # #)) (#) ?))
    222:29 16 (map1 (((gnu packages linux)) ((gnu packages #)) ((?)) ?))
    222:29 15 (map1 (((gnu packages pkg-config)) ((gnu packages #)) ?))
    222:29 14 (map1 (((gnu packages python)) ((gnu packages qt)) (#) ?))
    222:29 13 (map1 (((gnu packages qt)) ((gnu packages xdisorg)) # #))
    222:29 12 (map1 (((gnu packages xdisorg)) ((gnu packages xfce)) #))
    222:17 11 (map1 (((gnu packages xfce)) ((gnu packages xorg))))
   3327:17 10 (resolve-interface (gnu packages xfce) #:select _ #:hide ?)
In ice-9/threads.scm:
     390:8  9 (_ _)
In ice-9/boot-9.scm:
   3253:13  8 (_)
In ice-9/threads.scm:
     390:8  7 (_ _)
In ice-9/boot-9.scm:
   3544:20  6 (_)
    2836:4  5 (save-module-excursion #<procedure 7fdaa6fff630 at ice-?>)
   3564:26  4 (_)
In unknown file:
            3 (primitive-load-path "gnu/packages/xfce" #<procedure 7f?>)
In gnu/packages/xfce.scm:
   1156:19  2 (_)
In ice-9/boot-9.scm:
   1685:16  1 (raise-exception _ #:continuable? _)
   1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
error: mate-polkit: unbound variable

$ guix describe
   guix af95f2d
     bewaarplaats-URL: https://git.savannah.gnu.org/git/guix.git
     tak: master
     commit: af95f2d8f98eb2c8c64954bb2fd0b70838899174
   nonguix 110f914
     bewaarplaats-URL: https://gitlab.com/nonguix/nonguix
     tak: master
     commit: 110f914f81d0d2c67a45f9b2f87a5dc7ec54dfd0

The backtrace only mentions Guix modules, so the bug doesn't appear to 
be in the extra channel should be irrelevant.

gnu/packages/xfce.scm: 1156:19:

[... mate-screenscraver ...]
     (build-system glib-or-gtk-build-system)
     (arguments
      `(#:configure-flags
        ;; FIXME: There is a permissions problem with screen locking
        ;; which effectively locks you out completely. Enable locking
        ;; once this has been fixed.
        (list "--enable-locking" "--with-kbd-layout-indicator"
              "--with-xf86gamma-ext" "--enable-pam"
              "--disable-schemas-compile" "--without-console-kit");L1156
        #:phases
        (modify-phases %standard-phases
          (add-after 'unpack 'autoconf
            (lambda* (#:key outputs #:allow-other-keys)
              (let* ((out (assoc-ref outputs "out"))
                     (dbus-dir (string-append out 
"/share/dbus-1/services")))
              (setenv "SHELL" (which "sh"))
              (setenv "CONFIG_SHELL" (which "sh"))
              (substitute* "configure"
                (("dbus-1") ""))))))))

Something is screwy with the line number information too, as 
mate-screensaver doesn't mention mate-polkit.

Greetings,
Maxime.