Fix cextern helloworld

cextern/helloworld.c

 // Simple C program to display "Hello World"
 
 // Header file for input output functions
-#include <stdio.h>
+#include <Python.h>
 
-// main function -
-// where the execution of program begins
-int main()
+static PyObject* _helloworld(PyObject* self, PyObject* args)
 {
+    printf("Hello World\n");
+    return Py_None;
+}
+
+static PyMethodDef Methods[] = {
+    { "helloworld", _helloworld, METH_NOARGS, "Prints Hello World" },
+    { NULL, NULL, 0, NULL }
+};
 
-    // prints hello world
-    printf("Hello World");
+// Our Module Definition struct
+static struct PyModuleDef Module = {
+    PyModuleDef_HEAD_INIT,
+    "helloworld",
+    "prints Hello World",
+    -1,
+    Methods
+};
 
-    return 0;
+// Initializes our module using our above struct
+PyMODINIT_FUNC PyInit_helloworld(void)
+{
+    return PyModule_Create(&Module);
 }