Hide warnings from Python headers when using gcc-compatible compiler.

Like commit 388e80132, use "#pragma GCC system_header" to silence
warnings appearing within the Python headers, since newer Python
versions no longer worry about some restrictions we still use like
-Wdeclaration-after-statement.

This patch improves on 388e80132 by inventing a separate wrapper
header file, allowing the pragma to be tightly scoped to just
the Python headers and not other stuff we have laying about in
plpython.h.  I applied the same technique to plperl for the same
reason: the original patch suppressed warnings for a good deal
of our own code, not only the Perl headers.

Like the previous commit, back-patch to supported branches.

Peter Eisentraut and Tom Lane

Discussion: https://postgr.es/m/ae523163-6d2a-4b81-a875-832e48dec502@eisentraut.org

src/pl/plpython/plpython_system.h

@@ -0,0 +1,53 @@
+/*-------------------------------------------------------------------------
+ *
+ * plpython_system.h - pull in Python's system header files
+ *
+ * We break this out as a separate header file to precisely control
+ * the scope of the "system_header" pragma.  No Postgres-specific
+ * declarations should be put here.  However, we do include some stuff
+ * that is meant to prevent conflicts between our code and Python.
+ *
+ * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/pl/plpython/plpython_system.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PLPYTHON_SYSTEM_H
+#define PLPYTHON_SYSTEM_H
+
+/*
+ * Newer versions of the Python headers trigger a lot of warnings with our
+ * preferred compiler flags (at least -Wdeclaration-after-statement is known
+ * to be problematic). The system_header pragma hides warnings from within
+ * the rest of this file, if supported.
+ */
+#ifdef HAVE_PRAGMA_GCC_SYSTEM_HEADER
+#pragma GCC system_header
+#endif
+
+/*
+ * Python versions <= 3.8 otherwise define a replacement, causing macro
+ * redefinition warnings.
+ */
+#define HAVE_SNPRINTF 1
+
+#if defined(_MSC_VER) && defined(_DEBUG)
+/* Python uses #pragma to bring in a non-default libpython on VC++ if
+ * _DEBUG is defined */
+#undef _DEBUG
+/* Also hide away errcode, since we load Python.h before postgres.h */
+#define errcode __msvc_errcode
+#include <Python.h>
+#undef errcode
+#define _DEBUG
+#elif defined (_MSC_VER)
+#define errcode __msvc_errcode
+#include <Python.h>
+#undef errcode
+#else
+#include <Python.h>
+#endif
+
+#endif							/* PLPYTHON_SYSTEM_H */