plpython: Stop undefining _POSIX_C_SOURCE, _XOPEN_SOURCE

We undefined them to avoid warnings about macro redefinitions. But we haven't
fully followed the necessary include order, since at least 147c248254, in
2011.  Recently the combination of the include order rules not being followed
and undefining _POSIX_C_SOURCE started to cause a compile failure, starting
with 03023a2664. Undefining _POSIX_C_SOURCE hides clock_gettime(), which is
referenced in an inline function as of 03023a2664, whereas it was a macro
before.

After seeing some evidence that undefining _POSIX_C_SOURCE et al isn't
required, I tried to build postgres with plpython on most of our supported
platforms (except DragonFlyBSD and Illumos, but similar systems were tested),
with/without the #undefines. No compiler warning / behavioral difference.

The oldest supported python version, 3.2, defines _POSIX_C_SOURCE to 200112L
ad _XOPEN_SOURCE to 600, whereas newer versions of python use 200809L/700
respectively. As _POSIX_C_SOURCE/_XOPEN_SOURCE will default to the newer
operating system on most platforms, it's possible that when using python 3.2
new warnings would be emitted - but that seems acceptable.

It's possible that this approach won't work on some older platforms. But
getting rid of most of the include-order complexity seems promising, and it's
an easily revertible patch if we end up having to go another way.

Discussion: https://postgr.es/m/20230124165814.2njc7gnvubn2amh6@awork3.anarazel.de

src/pl/plpython/plpython.h

@@ -12,23 +12,13 @@
 #ifndef PLPYTHON_H
 #define PLPYTHON_H
 
-/*
- * Include order should be: postgres.h, other postgres headers, plpython.h,
- * other plpython headers.  (In practice, other plpython headers will also
- * include this file, so that they can compile standalone.)
- */
-#ifndef POSTGRES_H
+/* postgres.h needs to be included before Python.h, as usual */
+#if !defined(POSTGRES_H)
 #error postgres.h must be included before plpython.h
+#elif defined(Py_PYTHON_H)
+#error Python.h must be included via plpython.h
 #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.
- */
-#undef _POSIX_C_SOURCE
-#undef _XOPEN_SOURCE
-
 /*
  * Python versions <= 3.8 otherwise define a replacement, causing macro
  * redefinition warnings.