scons: Reorder options for clarity

Also prefix all thirdparty-related toggles with `builtin`.

drivers/gl_context/SCsub

@@ -4,7 +4,7 @@ Import('env')
 
 if (env["platform"] in ["haiku", "osx", "windows", "x11"]):
     # Thirdparty source files
-    if (env["glew"] != "system"):  # builtin
+    if (env['builtin_glew'] != 'no'):  # builtin
         thirdparty_dir = "#thirdparty/glew/"
         thirdparty_sources = [
             "glew.c",