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);
ble>
<div class='path'>path: <a href='/guix/log/?id=24155bf40db2dfd25c167f41452ea0227a712654'>root</a>/<a href='/guix/log/tests?id=24155bf40db2dfd25c167f41452ea0227a712654'>tests</a>/<a href='/guix/log/tests/derivations.scm?id=24155bf40db2dfd25c167f41452ea0227a712654'>derivations.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/tests/derivations.scm?id=24155bf40db2dfd25c167f41452ea0227a712654&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-11-28 10:39:58 +0100'>2018-11-28</span></td><td><a href='/guix/commit/tests/derivations.scm?id=8856f409d13cd7376be4319b9f75df0692c009d6'>derivations: Add properties.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-09-04 17:25:11 +0200'>2018-09-04</span></td><td><a href='/guix/commit/tests/derivations.scm?id=ca719424455465fca4b872c371daf2a46de88b33'>Switch to Guile-Gcrypt.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-01-30 22:24:06 +0100'>2018-01-30</span></td><td><a href='/guix/commit/tests/derivations.scm?id=9d6fae65f3c2197e7b95afebd0a13908bdb4aed9'>derivations: Adjust tests for Stow environments.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-10-12 23:47:48 +0200'>2017-10-12</span></td><td><a href='/guix/commit/tests/derivations.scm?id=6ea10db973d861cd8774938e40151c0f8b2d266f'>tests: Support multiple HTTP server instances.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-06-02 18:47:06 +0200'>2017-06-02</span></td><td><a href='/guix/commit/tests/derivations.scm?id=2dc98729afb62e48b5866f599df9a9274d440686'>derivations: 'derivation-prerequisites-to-build' returns &lt;substitutable&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-06-02 18:47:06 +0200'>2017-06-02</span></td><td><a href='/guix/commit/tests/derivations.scm?id=ef51ac21eec28de3b0fb693f88be5f3c494d464a'>derivations: 'substitution-oracle' returns a &lt;substitutable&gt;.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-04-22 14:07:46 +0200'>2017-04-22</span></td><td><a href='/guix/commit/tests/derivations.scm?id=8a8e2d2ed5932a13a73583d32b152133d28aedf5'>derivations: Adjust builder encoding test.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-04-21 22:45:29 +0200'>2017-04-21</span></td><td><a href='/guix/commit/tests/derivations.scm?id=9231ef12f2a595b8f1e677dbe50cc499555302b6'>derivations: Restore UTF-8 encoding of build scripts.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-03-28 10:00:05 +0200'>2017-03-28</span></td><td><a href='/guix/commit/tests/derivations.scm?id=bdb59b331bac0dea4a75b055334313ddc7bfecc8'>derivations: Do not fetch narinfos for non-substitutable items.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-11 17:06:31 +0100'>2017-01-11</span></td><td><a href='/guix/commit/tests/derivations.scm?id=9b5364a3afb03414bd6e3ded2fbfdacabe4e8870'>daemon: Allow check builds of 'builtin:download' derivations.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-11-16 18:19:54 +0100'>2016-11-16</span></td><td><a href='/guix/commit/tests/derivations.scm?id=f9aefa2d5fb3f6aad25a907939ee872c828b33d0'>daemon: Add 'built-in-builders' RPC.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-11-16 18:19:47 +0100'>2016-11-16</span></td><td><a href='/guix/commit/tests/derivations.scm?id=94d92c7796a3dd50c27d532315f7d497ac99f08e'>daemon: Add "builtin:download" derivation builder.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>