gnu: phodav: Fix udev rules directory.

Since commit da7a5e3597 (e)udev is
actually found triggering the installation of phodav's udev rules.

That's great, except that it uses (e)udev's 'udevdir' pkg-config
variable for that, which is of course not writable.

* gnu/packages/gnome.scm (phodav)[arguments]: Add a new
'fix-udev-rules-directory phase.

gnu/packages/gnome.scm

@@ -1167,13 +1167,19 @@ (define-public phodav
    (arguments
     `(#:phases
       (modify-phases %standard-phases
+        (add-after 'unpack 'fix-udev-rules-directory
+          (lambda* (#:key outputs #:allow-other-keys)
+            (let* ((out   (assoc-ref outputs "out"))
+                   (rules (string-append out "/lib/udev/rules.d")))
+              (substitute* "data/meson.build"
+                (("udev\\.get_pkgconfig_variable\\('udevdir'\\)")
+                 (format #f "'~a'" rules))))))
         (add-before 'check 'start-virtual-dir-server
           ;; The same server when started by tests/virtual-dir returns an
           ;; unexpected status (4 instead of 200) and fails a test.  It is
           ;; unclear why starting it manually here makes it pass.
           (lambda _
-            (system "tests/virtual-dir-server &")
-            #t)))))
+            (system "tests/virtual-dir-server &"))))))
    (native-inputs
     `(("docbook-xml" ,docbook-xml-4.3)
       ("gettext" ,gettext-minimal)