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);
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=b38d893b909ebcf8f8fa3a53cd9378c49122a2e7'>root</a>/<a href='/guix/log/gnu?id=b38d893b909ebcf8f8fa3a53cd9378c49122a2e7'>gnu</a>/<a href='/guix/log/gnu/system?id=b38d893b909ebcf8f8fa3a53cd9378c49122a2e7'>system</a>/<a href='/guix/log/gnu/system/keyboard.scm?id=b38d893b909ebcf8f8fa3a53cd9378c49122a2e7'>keyboard.scm</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/keyboard.scm?id=b38d893b909ebcf8f8fa3a53cd9378c49122a2e7&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-04-06 00:14:13 +0200'>2020-04-06</span></td><td><a href='/guix/commit/gnu/system/keyboard.scm?id=2729cb406d9f3e9eb22cc660f887138134edb3a8'>system: Allow for comma-separated keyboard layouts.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-03-24 23:06:11 +0100'>2019-03-24</span></td><td><a href='/guix/commit/gnu/system/keyboard.scm?id=db1e2522f6222594fc507ce7a7ba7b1c0ac5037d'>Add (gnu system keyboard).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>