tkgl: try to fix build on darwin

Author: alejo7797

pkgs/by-name/tk/tkgl/package.nix

@@ -29,6 +29,10 @@ tcl.mkTclDerivation {
     "--with-tkinclude=${lib.getDev tk}/include"
   ];
 
+  env.NIX_CFLAGS_COMPILE = lib.optionals stdenv.hostPlatform.isDarwin [
+    "-Wno-error=unguarded-availability"
+  ];
+
   installTargets = [
     "install-lib-binaries"
   ];