gnu: gnumach-headers: Cross-build without relying on x86.

* gnu/packages/hurd.scm (gnumach-headers): Remove configure flags where we
say the builder is running x86.
* gnu/packages/cross-base.scm (cross-gnumach-headers): Work around limitation
of build system, by manually changing the target host_cpu without relying on
--host, because we don't have a working cross-compiler yet.

gnu/packages/cross-base.scm

@@ -375,6 +375,24 @@ (define* (cross-gnumach-headers target
     (inherit gnumach-headers)
     (name (string-append (package-name gnumach-headers)
                          "-cross-" target))
+    (arguments
+     (substitute-keyword-arguments (package-arguments gnumach-headers)
+       ((#:phases phases #~%standard-phases)
+        #~(modify-phases #$phases
+            ;; Cheat by setting the host_cpu variable manually, since using
+            ;; --host= would require a working cross-compiler, which we don't
+            ;; have yet.
+            (add-after 'unpack 'substitute-host-cpu
+              (lambda _
+                (substitute* "configure.ac"
+                  (("AC_CANONICAL_HOST")
+                   #$(string-append
+                      "host_cpu="
+                      (match target
+                        ((? target-x86-32?)
+                         "i386")
+                        ((? target-x86-64?)
+                         "x86_64")))))))))))
     (native-inputs
      (modify-inputs (package-native-inputs gnumach-headers)
        (prepend xgcc xbinutils)))))

gnu/packages/hurd.scm

@@ -6,6 +6,7 @@
 ;;; Copyright © 2020, 2022 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
 ;;; Copyright © 2020 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2020 Rene Saavedra <pacoon@protonmail.com>
+;;; Copyright © 2023 Josselin Poiret <dev@jpoiret.xyz>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -75,16 +76,6 @@ (define-public gnumach-headers
            (lambda _
              (invoke "make" "install-data")))
          (delete 'build))
-
-       ;; GNU Mach supports only IA32 currently, so cheat so that we can at
-       ;; least install its headers.
-       ,@(if (%current-target-system)
-             '()
-             ;; See <http://lists.gnu.org/archive/html/bug-hurd/2015-06/msg00042.html>
-             ;; <http://lists.gnu.org/archive/html/guix-devel/2015-06/msg00716.html>
-             '(#:configure-flags '("--build=i586-pc-gnu"
-                                   "--host=i686-linux-gnu")))
-
        #:tests? #f))
     (native-inputs
      (list autoconf automake texinfo-4))