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/dtoa.h

@@ -1,8 +1,7 @@
 #ifndef Py_LIMITED_API
 #ifndef PY_NO_SHORT_FLOAT_REPR
-#ifdef __cplusplus
-extern "C" {
-#endif
+COSMOPOLITAN_C_START_
+/* clang-format off */
 
 PyAPI_FUNC(double) _Py_dg_strtod(const char *str, char **ptr);
 PyAPI_FUNC(char *) _Py_dg_dtoa(double d, int mode, int ndigits,
@@ -11,9 +10,6 @@ PyAPI_FUNC(void) _Py_dg_freedtoa(char *s);
 PyAPI_FUNC(double) _Py_dg_stdnan(int sign);
 PyAPI_FUNC(double) _Py_dg_infinity(int sign);
 
-
-#ifdef __cplusplus
-}
-#endif
+COSMOPOLITAN_C_END_
 #endif
 #endif