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);
x/log/?id=a0121643d617bd69df3dc9486a58e183ae726cf7'>root</a>/<a href='/guix/log/etc?id=a0121643d617bd69df3dc9486a58e183ae726cf7'>etc</a>/<a href='/guix/log/etc/substitutes?id=a0121643d617bd69df3dc9486a58e183ae726cf7'>substitutes</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/etc/substitutes?id=a0121643d617bd69df3dc9486a58e183ae726cf7&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-07-10 00:11:00 +0200'>2020-07-10</span></td><td><a href='/guix/commit/etc/substitutes?id=d283bb960f927dd5f7bb8b96bc697221e4e8ad39'>maint: Remove traces of "berlin.guixsd.org".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-06-29 14:27:22 +0200'>2019-06-29</span></td><td><a href='/guix/commit/etc/substitutes?id=3a8bfebed94ba295432da8304b7ffb4a611ef761'>etc: Remove hydra.gnu.org.pub key.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-05-01 11:05:47 +0200'>2019-05-01</span></td><td><a href='/guix/commit/etc/substitutes?id=757e633d57a7b994759d69e41dd759a39f1a34ae'>build: Change default substitute server to "ci.guix.gnu.org".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-12-04 10:57:56 +0100'>2018-12-04</span></td><td><a href='/guix/commit/etc/substitutes?id=6a837b6062ab405f6ca6d9c0163836769098d89b'>etc: Add "ci.guix.info.pub" public key file.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-11-06 00:04:11 +0100'>2018-11-06</span></td><td><a href='/guix/commit/etc/substitutes?id=aa919fbc890c5438166083f9ff02e4b1968a182b'>maint: Move substitute public keys to etc/substitutes.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>