libGLU: fix on darwin

Fixes a configuration issue:

meson.build:33:8: ERROR: requires argument not a string, library with pkgconfig-generated file or pkgconfig-dependency object, got <ExtraFrameworkDependency opengl: True>

pkgs/development/libraries/mesa-glu/default.nix

···
 
   outputs = [ "out" "dev" ];
 
+  mesonFlags = lib.optionals stdenv.isDarwin [
+    "-Dgl_provider=gl" # glvnd is default
+  ];
+
   enableParallelBuilding = true;
 
   passthru = {