Issue #22463: Backport compiler warning fixes and workarounds

* Set but unused variable in Parser/pgen.c in non-debug builds. Patch by
  Christian Heimes.
* Unused static function in Modules/readline.c. Patch by Georg Brandl.
* main_window unused in Modules/tkappinit.c. Patch by Gregory P. Smith.
* Dead assignment in Modules/_ctypes/cfield.c. Extracted from patch by Brett
  Cannon.
* Expression result unused in PyObject_INIT macro expansions. Based on
  patches by Christian Heimes.
* Load expat_config.h and therefore pyconfig.h before C stdlib headers are
  loaded. This silences pre-processor warnings including '_POSIX_C_SOURCE
  redefined'. Extracted from patch by Christian Heimes.
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index f2db6da..1a04b78 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -88,7 +88,7 @@
     op = (PyStringObject *)PyObject_MALLOC(PyStringObject_SIZE + size);
     if (op == NULL)
         return PyErr_NoMemory();
-    PyObject_INIT_VAR(op, &PyString_Type, size);
+    (void)PyObject_INIT_VAR(op, &PyString_Type, size);
     op->ob_shash = -1;
     op->ob_sstate = SSTATE_NOT_INTERNED;
     if (str != NULL)
@@ -143,7 +143,7 @@
     op = (PyStringObject *)PyObject_MALLOC(PyStringObject_SIZE + size);
     if (op == NULL)
         return PyErr_NoMemory();
-    PyObject_INIT_VAR(op, &PyString_Type, size);
+    (void)PyObject_INIT_VAR(op, &PyString_Type, size);
     op->ob_shash = -1;
     op->ob_sstate = SSTATE_NOT_INTERNED;
     Py_MEMCPY(op->ob_sval, str, size+1);
@@ -1061,7 +1061,7 @@
     op = (PyStringObject *)PyObject_MALLOC(PyStringObject_SIZE + size);
     if (op == NULL)
         return PyErr_NoMemory();
-    PyObject_INIT_VAR(op, &PyString_Type, size);
+    (void)PyObject_INIT_VAR(op, &PyString_Type, size);
     op->ob_shash = -1;
     op->ob_sstate = SSTATE_NOT_INTERNED;
     Py_MEMCPY(op->ob_sval, a->ob_sval, Py_SIZE(a));
@@ -1103,7 +1103,7 @@
     op = (PyStringObject *)PyObject_MALLOC(PyStringObject_SIZE + nbytes);
     if (op == NULL)
         return PyErr_NoMemory();
-    PyObject_INIT_VAR(op, &PyString_Type, size);
+    (void)PyObject_INIT_VAR(op, &PyString_Type, size);
     op->ob_shash = -1;
     op->ob_sstate = SSTATE_NOT_INTERNED;
     op->ob_sval[size] = '\0';