Python: fixing Python 3.8 build with clang.

Python 3.8 has 'tp_print' field in PyTypeObject struct.  This field is
attributed as deprecated.  So, clang generates warning (which is turned to
error) as a result of initializing this field.  From the other hand, it is
impossible to omit this field in positional initialization.  The solution
is to use designated initializer.

Silencing usage message during configure python.

This is related to #331 issue on GitHub.

auto/modules/python

@@ -64,7 +64,7 @@ nxt_found=no
 
 if /bin/sh -c "$NXT_PYTHON_CONFIG --prefix" >> $NXT_AUTOCONF_ERR 2>&1; then
 
-    if ${NXT_PYTHON_CONFIG} --embed 2>/dev/null; then
+    if ${NXT_PYTHON_CONFIG} --embed >/dev/null 2>&1; then
         NXT_PYTHON_CONFIG="${NXT_PYTHON_CONFIG} --embed"
     fi