Perform build and magnum tuning

Building o//third_party/python now takes 5 seconds on my PC

This change works towards modifying Python to use runtime dispatching
when appropriate. For example, when loading the magnums in the socket
module, it's a good idea to check if the magnum is zero, because that
means the local system platform doesn't support it.

third_party/python/Include/ast.h

@@ -1,9 +1,9 @@
 #ifndef Py_AST_H
 #define Py_AST_H
-#ifdef __cplusplus
-extern "C" {
-#endif
+COSMOPOLITAN_C_START_
+/* clang-format off */
 
+extern int foo;
 PyAPI_FUNC(int) PyAST_Validate(mod_ty);
 PyAPI_FUNC(mod_ty) PyAST_FromNode(
     const node *n,
@@ -16,7 +16,5 @@ PyAPI_FUNC(mod_ty) PyAST_FromNodeObject(
     PyObject *filename,
     PyArena *arena);
 
-#ifdef __cplusplus
-}
-#endif
+COSMOPOLITAN_C_END_
 #endif /* !Py_AST_H */