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
e04'>commit</a><a href='/guix/diff/tests/services.scm?id=f22c20e7ca14f79f0be5ce228f55d934cda27e04'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/services.scm'>
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<div class='path'>path: <a href='/guix/log/?id=f22c20e7ca14f79f0be5ce228f55d934cda27e04'>root</a>/<a href='/guix/log/tests?id=f22c20e7ca14f79f0be5ce228f55d934cda27e04'>tests</a>/<a href='/guix/log/tests/services.scm?id=f22c20e7ca14f79f0be5ce228f55d934cda27e04'>services.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/services.scm?id=f22c20e7ca14f79f0be5ce228f55d934cda27e04&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-06-06 11:54:39 +0200'>2023-06-06</span></td><td><a href='/guix/commit/tests/services.scm?id=181951207339508789b28ba7cb914f983319920f'>services: 'modify-services' preserves service ordering.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-06-02 16:16:12 +0200'>2023-06-02</span></td><td><a href='/guix/commit/tests/services.scm?id=ae707b62e71b1fae054eb422412384bcc8d39fa9'>tests: Check for service existence in MODIFY-SERVICES</a><span class='msg-avail'>...</span></td><td>Brian Cully</td></tr>
<tr><td><span title='2023-06-02 16:16:07 +0200'>2023-06-02</span></td><td><a href='/guix/commit/tests/services.scm?id=09fcf261c1f66a07891ec2fe80f8652fd286bd3e'>tests: Add tests for MODIFY-SERVICES procedure</a><span class='msg-avail'>...</span></td><td>Brian Cully</td></tr>