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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<tr><td><span title='2023-09-17 02:00:00 +0200'>2023-09-17</span></td><td><a href='/guix/commit/gnu/packages/opencl.scm?id=71ec12d8bacd3901e8c7853d3c9403f3d09dfb31'>gnu: ocl-icd: Delete dubiously deprecated duplicate definition.</a><span class='msg-avail'>...<span class='msg-tooltip'>This follows up on commit 3341b22f594398b3aa3ff930d99b74a78abd1a30 which
re-added ocl-icd.  opencl-icd-loader is a different, incompatible package.

* gnu/packages/opencl.scm (ocl-icd): Remove duplicate variable.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2023-09-10 02:00:00 +0200'>2023-09-10</span></td><td><a href='/guix/commit/gnu/packages/opencl.scm?id=40e78353a4699b8edb6600d82df648914c9110b9'>Revert "gnu: beignet: Fix build."</a><span class='msg-avail'>...<span class='msg-tooltip'>This reverts commit 213516f0baf92b6f50bd0c2e005625c0b3421a6f,
which removed ICD support entirely.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2023-09-10 02:00:00 +0200'>2023-09-10</span></td><td><a href='/guix/commit/gnu/packages/opencl.scm?id=1ad4d00a9522289b94bb230674ffb585f1dcf417'>gnu: beignet: Fix build.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/opencl.scm (beignet)[inputs]: Add ocl-icd.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2023-09-10 02:00:00 +0200'>2023-09-10</span></td><td><a href='/guix/commit/gnu/packages/opencl.scm?id=3341b22f594398b3aa3ff930d99b74a78abd1a30'>gnu: Add ocl-icd.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/opencl.scm (ocl-icd): New public variable.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2023-09-15 22:23:44 +0200'>2023-09-15</span></td><td><a href='/guix/commit/gnu/packages/opencl.scm?id=213516f0baf92b6f50bd0c2e005625c0b3421a6f'>gnu: beignet: Fix build.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/opencl.scm (beignet)[arguments]: Add configure flag.
</span></span></td><td>Andreas Enge</td></tr>