xdg-utils: Support cross-compilation.

It has been verified that this does not cause rebuilds when compiling
natively.  The references graph when cross-compiling has also been verified --
glibc-2.33 and the native bash-static-5.1.8 still remains in the graph, but
via the cross-compiled inetutils-2.0, ncurses-6.2.20210619 and via
gcc-cross-TARGET-10.3.0-lib, which is not related with Perl cross-compilation.

* gnu/packages/freedesktop.scm
(xdg-utils)[inputs]{bash-minimal,file}: New inputs when cross-compiling.
(xdg-utils)[arguments]<#:phases>{locate-catalog-files}: Add 'native-inputs'
argument when cross-compiling. Look for docbook-xml and docbook-xsl in
native-inputs when cross-compiling.  While we are at it, eliminate input
labels with search-input-file.
(xdg-utils)[arguments]<#:phases>{patch-hardcoded-patch}: Use
search-input-file + inputs instead of 'which' when cross-compiling.

Signed-off-by: Mathieu Othacehe <othacehe@gnu.org>

gnu/packages/freedesktop.scm

@@ -25,7 +25,7 @@
 ;;; Copyright © 2021 pineapples <guixuser6392@protonmail.com>
 ;;; Copyright © 2021 Sarah Morgensen <iskarian@mgsn.dev>
 ;;; Copyright © 2021 Robby Zambito <contact@robbyzambito.me>
-;;; Copyright © 2021 Maxime Devos <maximedevos@telenet.be>
+;;; Copyright © 2021, 2022 Maxime Devos <maximedevos@telenet.be>
 ;;; Copyright © 2021 John Kehayias <john.kehayias@protonmail.com>
 ;;; Copyright © 2021, 2021 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2022 Daniel Meißner <daniel.meissner-i4k@ruhr-uni-bochum.de>
@@ -76,6 +76,7 @@
   #:use-module (gnu packages disk)
   #:use-module (gnu packages docbook)
   #:use-module (gnu packages documentation)
+  #:use-module (gnu packages file)
   #:use-module (gnu packages fontutils)
   #:use-module (gnu packages gawk)
   #:use-module (gnu packages gettext)
@@ -414,7 +415,15 @@ inappropriate content.")
      (list docbook-xsl docbook-xml-4.1.2 libxslt w3m xmlto))
     (inputs
      `(("awk" ,gawk)
+       ;; TODO(staging): Make this unconditional, to avoid canonical packages,
+       ;; see <https://lists.gnu.org/archive/html/guix-devel/2020-02/msg00148.html>.
+       ,@(if (%current-target-system)
+             `(("bash-minimal" ,bash-minimal)) ; for 'wrap-program'
+             '())
        ("coreutils" ,coreutils)
+       ,@(if (%current-target-system)
+             `(("file" ,file))
+             '())
        ("grep" ,grep)
        ("inetutils" ,inetutils) ; xdg-screensaver uses `hostname'
        ("perl-file-mimeinfo" ,perl-file-mimeinfo) ; for mimeopen fallback
@@ -428,19 +437,44 @@ inappropriate content.")
        #:phases
        (modify-phases %standard-phases
          (add-after 'unpack 'patch-hardcoded-paths
-           (lambda _
+           ;; TODO(staging): make unconditional
-             (substitute* "scripts/xdg-mime.in"
+           (,@(if (%current-target-system)
-               (("/usr/bin/file") (which "file")))
+                 '(lambda* (#:key inputs #:allow-other-keys))
-             (substitute* "scripts/xdg-open.in"
+                 '(lambda _))
-               (("/usr/bin/printf") (which "printf")))
+            (substitute* "scripts/xdg-mime.in"
-             #t))
+              (("/usr/bin/file")
+               (,@(if (%current-target-system)
+                      '(search-input-file inputs "bin/file")
+                      '(which "file")))))
+            (substitute* "scripts/xdg-open.in"
+              (("/usr/bin/printf")
+               (,@(if (%current-target-system)
+                      '(search-input-file inputs "bin/printf")
+                      '(which "printf")))))
+            #t))
          (add-before 'build 'locate-catalog-files
-           (lambda* (#:key inputs #:allow-other-keys)
+           ;; TODO(staging): Make unconditional for simplicity.
-             (let ((xmldoc (string-append (assoc-ref inputs "docbook-xml")
+           (lambda* (#:key inputs ,@(if (%current-target-system)
-                                          "/xml/dtd/docbook"))
+                                        '(native-inputs)
-                   (xsldoc (string-append (assoc-ref inputs "docbook-xsl")
+                                        '()) #:allow-other-keys)
-                                          "/xml/xsl/docbook-xsl-"
+             ;; TODO(staging): Make unconditional for simplicity and
-                                          ,(package-version docbook-xsl))))
+             ;; to avoid depending on input labels.
+             (let ,(if (%current-target-system)
+                       `((native-inputs (or native-inputs inputs))
+                         (xmldoc (search-input-directory native-inputs
+                                                         "xml/dtd/docbook"))
+                         (xsldoc
+                          (search-input-directory
+                           native-inputs
+                           (string-append "xml/xsl/docbook-xsl-"
+                                          ,(package-version docbook-xsl)))))
+                       `((xmldoc
+                          (string-append (assoc-ref inputs "docbook-xml")
+                                         "/xml/dtd/docbook"))
+                         (xsldoc
+                          (string-append (assoc-ref inputs "docbook-xsl")
+                                         "/xml/xsl/docbook-xsl-"
+                                         ,(package-version docbook-xsl)))))
                (for-each (lambda (file)
                            (substitute* file
                              (("http://.*/docbookx\\.dtd")
@@ -456,6 +490,8 @@ inappropriate content.")
                                  "/manpages/docbook.xsl man")))
                (setenv "STYLESHEET"
                        (string-append xsldoc "/html/docbook.xsl"))
+               ;; TODO(staging): Might as well remove the #t while we are at
+               ;; it.
                #t)))
          (add-after 'install 'wrap-executables
            (lambda* (#:key inputs outputs #:allow-other-keys)