Make sure the module init function is declared as 'extern "C"' in C++ also...

Make sure the module init function is declared as 'extern "C"' in C++ also when the CYTHON_NO_PYINIT_EXPORT option is used.
Closes #3414.

CHANGES.rst

@@ -32,6 +32,10 @@ Bugs fixed
   additional command line arguments leaked into the regular command.
   Patch by Kamekameha.  (Github issue #2209)
 
+* When using the ``CYTHON_NO_PYINIT_EXPORT`` option in C++, the module init function
+  was not declared as ``extern "C"``.
+  (Github issue #3414)
+
 
 0.29.17 (2020-04-26)
 ====================

Cython/Utility/ModuleSetupCode.c

@@ -695,24 +695,24 @@ static CYTHON_INLINE void * PyThread_tss_get(Py_tss_t *key) {
 
 /////////////// PyModInitFuncType.proto ///////////////
 
-#if PY_MAJOR_VERSION < 3
+#ifndef CYTHON_NO_PYINIT_EXPORT
+#define __Pyx_PyMODINIT_FUNC PyMODINIT_FUNC
 
-#ifdef CYTHON_NO_PYINIT_EXPORT
-// define this to void manually because PyMODINIT_FUNC adds __declspec(dllexport) to it's definition.
-#define __Pyx_PyMODINIT_FUNC void
+#elif PY_MAJOR_VERSION < 3
+// Py2: define this to void manually because PyMODINIT_FUNC adds __declspec(dllexport) to it's definition.
+#ifdef __cplusplus
+#define __Pyx_PyMODINIT_FUNC extern "C" void
 #else
-#define __Pyx_PyMODINIT_FUNC PyMODINIT_FUNC
+#define __Pyx_PyMODINIT_FUNC void
 #endif
 
 #else
-
-#ifdef CYTHON_NO_PYINIT_EXPORT
-// define this to PyObject * manually because PyMODINIT_FUNC adds __declspec(dllexport) to it's definition.
-#define __Pyx_PyMODINIT_FUNC PyObject *
+// Py3+: define this to PyObject * manually because PyMODINIT_FUNC adds __declspec(dllexport) to it's definition.
+#ifdef __cplusplus
+#define __Pyx_PyMODINIT_FUNC extern "C" PyObject *
 #else
-#define __Pyx_PyMODINIT_FUNC PyMODINIT_FUNC
+#define __Pyx_PyMODINIT_FUNC PyObject *
 #endif
-
 #endif
 
 
@@ -858,7 +858,7 @@ static CYTHON_INLINE int __Pyx_PyErr_GivenExceptionMatches2(PyObject *err, PyObj
 /////////////// MathInitCode ///////////////
 
 #if defined(WIN32) || defined(MS_WINDOWS)
-  #define _USE_MATH_DEFINES
+  #define _USE_MATH_DEFINESg
 #endif
 #include <math.h>