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.h

@@ -22,113 +22,15 @@
 #endif
 
 /*
- * Undefine some things that get (re)defined in the Python headers. They aren't
- * used by the PL/Python code, and all PostgreSQL headers should be included
- * earlier, so this should be pretty safe.
+ * Pull in Python headers via a wrapper header, to control the scope of
+ * the system_header pragma therein.
  */
-#undef _POSIX_C_SOURCE
-#undef _XOPEN_SOURCE
-
-/*
- * Sometimes python carefully scribbles on our *printf macros.
- * So we undefine them here and redefine them after it's done its dirty deed.
- */
-#undef vsnprintf
-#undef snprintf
-#undef vsprintf
-#undef sprintf
-#undef vfprintf
-#undef fprintf
-#undef vprintf
-#undef printf
-
-#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
-
-/*
- * Python 2/3 strings/unicode/bytes handling.  Python 2 has strings
- * and unicode, Python 3 has strings, which are unicode on the C
- * level, and bytes.  The porting convention, which is similarly used
- * in Python 2.6, is that "Unicode" is always unicode, and "Bytes" are
- * bytes in Python 3 and strings in Python 2.  Since we keep
- * supporting Python 2 and its usual strings, we provide a
- * compatibility layer for Python 3 that when asked to convert a C
- * string to a Python string it converts the C string from the
- * PostgreSQL server encoding to a Python Unicode object.
- */
-#if PY_MAJOR_VERSION >= 3
-#define PyString_Check(x) 0
-#define PyString_AsString(x) PLyUnicode_AsString(x)
-#define PyString_FromString(x) PLyUnicode_FromString(x)
-#define PyString_FromStringAndSize(x, size) PLyUnicode_FromStringAndSize(x, size)
-#endif
-
-/*
- * Python 3 only has long.
- */
-#if PY_MAJOR_VERSION >= 3
-#define PyInt_FromLong(x) PyLong_FromLong(x)
-#define PyInt_AsLong(x) PyLong_AsLong(x)
-#endif
-
-/* Python 3 removed the Py_TPFLAGS_HAVE_ITER flag */
-#if PY_MAJOR_VERSION >= 3
-#define Py_TPFLAGS_HAVE_ITER 0
-#endif
+#include "plpython_system.h"
 
 /* define our text domain for translations */
 #undef TEXTDOMAIN
 #define TEXTDOMAIN PG_TEXTDOMAIN("plpython")
 
-/* put back our *printf macros ... this must match src/include/port.h */
-#ifdef vsnprintf
-#undef vsnprintf
-#endif
-#ifdef snprintf
-#undef snprintf
-#endif
-#ifdef vsprintf
-#undef vsprintf
-#endif
-#ifdef sprintf
-#undef sprintf
-#endif
-#ifdef vfprintf
-#undef vfprintf
-#endif
-#ifdef fprintf
-#undef fprintf
-#endif
-#ifdef vprintf
-#undef vprintf
-#endif
-#ifdef printf
-#undef printf
-#endif
-
-#define vsnprintf		pg_vsnprintf
-#define snprintf		pg_snprintf
-#define vsprintf		pg_vsprintf
-#define sprintf			pg_sprintf
-#define vfprintf		pg_vfprintf
-#define fprintf			pg_fprintf
-#define vprintf			pg_vprintf
-#define printf(...)		pg_printf(__VA_ARGS__)
-
 /*
  * Used throughout, and also by the Python 2/3 porting layer, so it's easier to
  * just include it everywhere.