Close #13415: Test in configure if unsetenv() has a return value or not.

Patch written by Charles-François Natali.

Modules/posixmodule.c

@@ -7841,19 +7841,24 @@ static PyObject *
 posix_unsetenv(PyObject *self, PyObject *args)
 {
     PyObject *name;
+#ifndef HAVE_BROKEN_UNSETENV
     int err;
+#endif
 
     if (!PyArg_ParseTuple(args, "O&:unsetenv",
 
                           PyUnicode_FSConverter, &name))
         return NULL;
 
-
+#ifdef HAVE_BROKEN_UNSETENV
+    unsetenv(PyBytes_AS_STRING(name));
+#else
     err = unsetenv(PyBytes_AS_STRING(name));
     if (err) {
         Py_DECREF(name);
         return posix_error();
     }
+#endif
 
     /* Remove the key from posix_putenv_garbage;
      * this will cause it to be collected.  This has to