python3Packages.glcontext: fix darwin build (#478997)

pkgs/development/python-modules/glcontext/default.nix

@@ -1,4 +1,5 @@
 {
+  stdenv,
   lib,
   buildPythonPackage,
   fetchFromGitHub,
@@ -26,7 +27,7 @@ buildPythonPackage rec {
     libX11
   ];
 
-  postPatch = ''
+  postPatch = lib.optionalString (stdenv.hostPlatform.isLinux) ''
     substituteInPlace glcontext/x11.cpp \
       --replace-fail '"libGL.so"' '"${libGL}/lib/libGL.so"' \
       --replace-fail '"libX11.so"' '"${libX11}/lib/libX11.so"'
@@ -46,7 +47,7 @@ buildPythonPackage rec {
     homepage = "https://github.com/moderngl/glcontext";
     description = "OpenGL implementation for ModernGL";
     license = lib.licenses.mit;
-    platforms = lib.platforms.linux;
+    platforms = lib.platforms.linux ++ lib.platforms.darwin;
     maintainers = [ ];
   };
 }