gnu: libgnomeui: Fix build with Python 3.6.

* gnu/packages/patches/libgnomeui-utf8.patch: New file.
* gnu/local.mk (dist_patch_DATA): Add it.
* gnu/packages/gnome.scm (libgnomeui)[source]: Use it.

gnu/local.mk

@@ -815,6 +815,7 @@ dist_patch_DATA =						\
   %D%/packages/patches/libgdata-fix-tests.patch			\
   %D%/packages/patches/libgdata-glib-duplicate-tests.patch	\
   %D%/packages/patches/libgnome-encoding.patch			\
+  %D%/packages/patches/libgnomeui-utf8.patch			\
   %D%/packages/patches/libgxps-CVE-2017-11590.patch		\
   %D%/packages/patches/libffi-3.2.1-complex-alpha.patch		\
   %D%/packages/patches/libjxr-fix-function-signature.patch	\

gnu/packages/gnome.scm

@@ -1601,6 +1601,7 @@ (define-public libgnomeui
               (uri (string-append "mirror://gnome/sources/" name "/"
                                   (version-major+minor version)  "/"
                                   name "-" version ".tar.bz2"))
+              (patches (search-patches "libgnomeui-utf8.patch"))
               (sha256
                (base32
                 "03rwbli76crkjl6gp422wrc9lqpl174k56cp9i96b7l8jlj2yddf"))))

gnu/packages/patches/libgnomeui-utf8.patch

@@ -0,0 +1,51 @@
+Fixes build with Python >= 3.6:
+
+------
+Making all in libgnomeui
+make[2]: Entering directory '/tmp/guix-build-libgnomeui-2.24.5.drv-0/libgnomeui-2.24.5/libgnomeui'
+  GEN    stamp-gnome-marshal.h
+  GEN    stamp-gnometypebuiltins.h
+INFO: Reading ./gnome-marshal.list...
+  GEN    gnome-marshal.h
+Traceback (most recent call last):
+  File "/gnu/store/azh1is0xknn4xphwj33iqcb5ic9qhk8l-glib-2.54.2-bin/bin/glib-mkenums", line 688, in <module>
+    process_file(fname)
+  File "/gnu/store/azh1is0xknn4xphwj33iqcb5ic9qhk8l-glib-2.54.2-bin/bin/glib-mkenums", line 420, in process_file
+    line = curfile.readline()
+  File "/gnu/store/3lkypf5wnsnvkaidhw0pv7k3yjfh1r9g-python-3.6.3/lib/python3.6/codecs.py", line 321, in decode
+    (result, consumed) = self._buffer_decode(data, self.errors, final)
+UnicodeDecodeError: 'utf-8' codec can't decode byte 0xf1 in position 1009: invalid continuation byte
+make[2]: *** [Makefile:1109: stamp-gnometypebuiltins.h] Error 1
+make[2]: Leaving directory '/tmp/guix-build-libgnomeui-2.24.5.drv-0/libgnomeui-2.24.5/libgnomeui'
+make[1]: *** [Makefile:369: all-recursive] Error 1
+make[1]: Leaving directory '/tmp/guix-build-libgnomeui-2.24.5.drv-0/libgnomeui-2.24.5'
+make: *** [Makefile:296: all] Error 2
+------
+
+Patch copied from upstream source repository:
+
+https://git.gnome.org/browse/libgnomeui/commit/?id=30334c28794ef85d8973f4ed0779b5ceed6594f2
+
+From 30334c28794ef85d8973f4ed0779b5ceed6594f2 Mon Sep 17 00:00:00 2001
+From: Colin Walters <walters@verbum.org>
+Date: Mon, 7 Aug 2017 13:15:26 -0400
+Subject: [PATCH] gnome-scores.h: Convert to UTF-8
+
+The new Python `glib-mkenums` barfs on ISO-8859-1 input.
+---
+ libgnomeui/gnome-scores.h | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/libgnomeui/gnome-scores.h b/libgnomeui/gnome-scores.h
+index 911b12d1..4e641d22 100644
+--- a/libgnomeui/gnome-scores.h
++++ b/libgnomeui/gnome-scores.h
+@@ -27,7 +27,7 @@
+  * "High Scores" Widget 
+  *
+  * AUTHOR: 
+- * Horacio J. Peña <horape@compendium.com.ar>
++ * Horacio J. Peña <horape@compendium.com.ar>
+  *
+  * This is free software (under the terms of the GNU LGPL)
+  *