mesa: fix build on Darwin

pkgs/development/libraries/mesa/darwin.patch

@@ -0,0 +1,17 @@
+diff --git a/src/glx/glxext.c b/src/glx/glxext.c
+index 8770863eb7c..537f0af112c 100644
+--- a/src/glx/glxext.c
++++ b/src/glx/glxext.c
+@@ -886,10 +886,11 @@ __glXInitialize(Display * dpy)
+    Bool zink = False;
+    Bool try_zink = False;
+ 
++   const char *env = getenv("MESA_LOADER_DRIVER_OVERRIDE");
++
+ #if defined(GLX_DIRECT_RENDERING) && (!defined(GLX_USE_APPLEGL) || defined(GLX_USE_APPLE))
+    Bool glx_direct = !debug_get_bool_option("LIBGL_ALWAYS_INDIRECT", false);
+    Bool glx_accel = !debug_get_bool_option("LIBGL_ALWAYS_SOFTWARE", false);
+-   const char *env = getenv("MESA_LOADER_DRIVER_OVERRIDE");
+ 
+    zink = env && !strcmp(env, "zink");
+    try_zink = False;

pkgs/development/libraries/mesa/default.nix

@@ -179,6 +179,11 @@ self = stdenv.mkDerivation {
 
   patches = [
     ./opencl.patch
+  ] ++ lib.optionals stdenv.isDarwin [
+    # Reorder things to make it build on Darwin again
+    # Submitted upstream: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29592
+    # FIXME: remove when merged or otherwise addressed
+    ./darwin.patch
   ];
 
   postPatch = ''