This patch makes GTK+ look for additional modules in a list of directories
specified by the environment variable "GUIX_GTK3_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	2015-09-20 20:09:05.060590217 +0200
+++ b/gtk/gtkmodules.c	2015-09-20 20:10:33.423124833 +0200
@@ -52,6 +52,7 @@
 get_module_path (void)
 {
   const gchar *module_path_env;
+  const gchar *module_guix_gtk3_path_env;
   const gchar *exe_prefix;
   gchar *module_path;
   gchar *default_dir;
@@ -61,6 +62,7 @@
     return result;
 
   module_path_env = g_getenv ("GTK_PATH");
+  module_guix_gtk3_path_env = g_getenv ("GUIX_GTK3_PATH");
   exe_prefix = g_getenv ("GTK_EXE_PREFIX");
 
   if (exe_prefix)
@@ -68,7 +70,13 @@
   else
     default_dir = g_build_filename (_gtk_get_libdir (), "gtk-3.0", NULL);
 
-  if (module_path_env)
+  if (module_guix_gtk3_path_env && module_path_env)
+    module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
+				module_guix_gtk3_path_env, module_path_env, default_dir, NULL);
+  else if (module_guix_gtk3_path_env)
+    module_path = g_build_path (G_SEARCHPATH_SEPARATOR_S,
+				module_guix_gtk3_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);
   else
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<div class='path'>path: <a href='/guix/log/?id=f7b1b22e4b0b5c6867bbfed362b0ebd4e5480507'>root</a>/<a href='/guix/log/gnu?id=f7b1b22e4b0b5c6867bbfed362b0ebd4e5480507'>gnu</a>/<a href='/guix/log/gnu/system?id=f7b1b22e4b0b5c6867bbfed362b0ebd4e5480507'>system</a>/<a href='/guix/log/gnu/system/examples?id=f7b1b22e4b0b5c6867bbfed362b0ebd4e5480507'>examples</a>/<a href='/guix/log/gnu/system/examples/asus-c201.tmpl?id=f7b1b22e4b0b5c6867bbfed362b0ebd4e5480507'>asus-c201.tmpl</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/system/examples/asus-c201.tmpl?id=f7b1b22e4b0b5c6867bbfed362b0ebd4e5480507&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-03-22 20:55:37 -0700'>2020-03-22</span></td><td><a href='/guix/commit/gnu/system/examples/asus-c201.tmpl?id=37f8c50cdc2e99189c74999319f3a1854d419546'>gnu: linux-libre-arm-veyron: Deprecate package.</a><span class='msg-avail'>...</span></td><td>Vagrant Cascadian</td></tr>