pyutil: silence a warning caused by Python.h

There is no great solution. Python.h expects to be included first and to be
allowed to pollute the define namespace. We don't want to do that, because in
other places we want to include sd-messages.h early, and also we want to use a
higher standard than the obsolete version that Python.h declares.
Unfortunately, there is no nice solution to this problem. So call #undef
enough times to silence warnings and after including the header, define the
standard for our code to a recent version.

Author: keszybz

src/systemd/pyutil.h

@@ -3,7 +3,17 @@
 #pragma once
 
 #define PY_SSIZE_T_CLEAN
-#include <Python.h>
+/* Work around bug in Python.h:
+ * it tries to redefine defines already defined by /usr/include/features.h,
+ * without calling #undef first, causing a warning to be emitted.
+ * (https://github.com/python/cpython/issues/61322). */
+#undef _POSIX_C_SOURCE
+#undef _XOPEN_SOURCE
+#  include <Python.h>
+#undef _POSIX_C_SOURCE
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE  800
+#define _POSIX_C_SOURCE 202405L
 
 void cleanup_Py_DECREFp(PyObject **p);
 PyObject* absolute_timeout(uint64_t t);