Avoid some C compiler warnings about constant conditions.

Cython/Utility/Coroutine.c

@@ -607,7 +607,7 @@ void __Pyx_Coroutine_ExceptionClear(__Pyx_ExcInfoStruct *exc_state) {
 #define __Pyx_Coroutine_AlreadyRunningError(gen)  (__Pyx__Coroutine_AlreadyRunningError(gen), (PyObject*)NULL)
 static void __Pyx__Coroutine_AlreadyRunningError(CYTHON_UNUSED __pyx_CoroutineObject *gen) {
     const char *msg;
-    if (0) {
+    if ((0)) {
     #ifdef __Pyx_Coroutine_USED
     } else if (__Pyx_Coroutine_Check((PyObject*)gen)) {
         msg = "coroutine already executing";
@@ -625,7 +625,7 @@ static void __Pyx__Coroutine_AlreadyRunningError(CYTHON_UNUSED __pyx_CoroutineOb
 #define __Pyx_Coroutine_NotStartedError(gen)  (__Pyx__Coroutine_NotStartedError(gen), (PyObject*)NULL)
 static void __Pyx__Coroutine_NotStartedError(CYTHON_UNUSED PyObject *gen) {
     const char *msg;
-    if (0) {
+    if ((0)) {
     #ifdef __Pyx_Coroutine_USED
     } else if (__Pyx_Coroutine_Check(gen)) {
         msg = "can't send non-None value to a just-started coroutine";

Cython/Utility/Optimize.c

@@ -600,7 +600,7 @@ static double __Pyx__PyObject_AsDouble(PyObject* obj); /* proto */
 static double __Pyx__PyObject_AsDouble(PyObject* obj) {
     PyObject* float_value;
 #if !CYTHON_USE_TYPE_SLOTS
-    float_value = PyNumber_Float(obj);  if (0) goto bad;
+    float_value = PyNumber_Float(obj);  if ((0)) goto bad;
 #else
     PyNumberMethods *nb = Py_TYPE(obj)->tp_as_number;
     if (likely(nb) && likely(nb->nb_float)) {