gnu: gtk+-2: Add patch to support GUIX_GTK2_PATH.

* gnu/packages/patches/gtk2-respect-GUIX_GTK2_PATH: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/gtk.scm (gtk+-2) [source]: Add patch.
[native-search-paths]: Add search path for GUIX_GTK2_PATH.

gnu-system.am

@@ -506,6 +506,7 @@ dist_patch_DATA =						\
   gnu/packages/patches/guile-present-coding.patch		\
   gnu/packages/patches/guile-relocatable.patch			\
   gnu/packages/patches/guile-rsvg-pkgconfig.patch		\
+  gnu/packages/patches/gtk2-respect-GUIX_GTK2_PATH.patch	\
   gnu/packages/patches/gtkglext-disable-disable-deprecated.patch \
   gnu/packages/patches/hop-bigloo-4.0b.patch			\
   gnu/packages/patches/hop-linker-flags.patch			\

gnu/packages/gtk.scm

@@ -522,7 +522,8 @@ (define-public gtk+-2
                                 name "-" version ".tar.xz"))
             (sha256
              (base32
-              "0mj6xn40py9r9lvzg633fal81xfwfm89d9mvz7jk4lmwk0g49imj"))))
+              "0mj6xn40py9r9lvzg633fal81xfwfm89d9mvz7jk4lmwk0g49imj"))
+            (patches (list (search-patch "gtk2-respect-GUIX_GTK2_PATH.patch")))))
    (build-system gnu-build-system)
    (outputs '("out" "doc"))
    (propagated-inputs
@@ -557,7 +558,11 @@ (define-public gtk+-2
          ;; FIXME: re-enable tests requiring an X server
          (substitute* "gtk/Makefile.in"
            (("SUBDIRS = theme-bits . tests") "SUBDIRS = theme-bits .")))
-      %standard-phases)))
+       %standard-phases)))
+   (native-search-paths
+    (list (search-path-specification
+           (variable "GUIX_GTK2_PATH")
+           (files '("lib/gtk-2.0")))))
    (synopsis "Cross-platform toolkit for creating graphical user interfaces")
    (description
     "GTK+, or the GIMP Toolkit, is a multi-platform toolkit for creating

gnu/packages/patches/gtk2-respect-GUIX_GTK2_PATH.patch

@@ -0,0 +1,46 @@
+This patch makes GTK+ look for additional modules in a list of directories
+specified by the environment variable "GUIX_GTK2_PATH".  This can be used
+instead of "GTK_PATH" to make GTK+ find modules that are incompatible with
+other major versions of GTK+.
+
+--- a/gtk/gtkmodules.c	2014-09-29 22:02:17.000000000 +0200
++++ b/gtk/gtkmodules.c	2015-12-02 18:41:53.306396938 +0100
+@@ -55,6 +55,7 @@
+ get_module_path (void)
+ {
+   const gchar *module_path_env;
++  const gchar *module_guix_gtk2_path_env;
+   const gchar *exe_prefix;
+   const gchar *home_dir;
+   gchar *home_gtk_dir = NULL;
+@@ -70,6 +71,7 @@
+     home_gtk_dir = g_build_filename (home_dir, ".gtk-2.0", NULL);
+ 
+   module_path_env = g_getenv ("GTK_PATH");
++  module_guix_gtk2_path_env = g_getenv ("GUIX_GTK2_PATH");
+   exe_prefix = g_getenv ("GTK_EXE_PREFIX");
+ 
+   if (exe_prefix)
+@@ -77,9 +79,21 @@
+   else
+     default_dir = g_build_filename (GTK_LIBDIR, "gtk-2.0", NULL);
+ 
+-  if (module_path_env && home_gtk_dir)
++  if (module_guix_gtk2_path_env && module_path_env && home_gtk_dir)
++    module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
++				module_guix_gtk2_path_env, module_path_env, home_gtk_dir, default_dir, NULL);
++  else if (module_guix_gtk2_path_env && home_gtk_dir)
++    module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
++				module_guix_gtk2_path_env, home_gtk_dir, default_dir, NULL);
++  else if (module_guix_gtk2_path_env && module_path_env)
++    module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
++				module_guix_gtk2_path_env, module_path_env, default_dir, NULL);
++  else if (module_path_env && home_gtk_dir)
+     module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
+ 				module_path_env, home_gtk_dir, default_dir, NULL);
++  else if (module_guix_gtk2_path_env)
++    module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
++				module_guix_gtk2_path_env, default_dir, NULL);
+   else if (module_path_env)
+     module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
+ 				module_path_env, default_dir, NULL);