Fix _remote_debugging header include order and macOS compatibility

The _GNU_SOURCE macro must be defined before any system headers are
included to enable GNU extensions like process_vm_readv. Moving it
before the extern "C" block ensures it takes effect. The internal
Python headers are also changed from angle brackets to quotes since
they're local to the project.

On macOS, the TARGET_OS_OSX macro may not be defined by older SDKs,
so we now include TargetConditionals.h explicitly and provide a
fallback definition when needed.

Author: pablogsal

Modules/_remote_debugging/_remote_debugging.h

@@ -8,23 +8,24 @@
 #ifndef Py_REMOTE_DEBUGGING_H
 #define Py_REMOTE_DEBUGGING_H
 
+/* _GNU_SOURCE must be defined before any system headers */
+#define _GNU_SOURCE
+
 #ifdef __cplusplus
 extern "C" {
 #endif
 
-#define _GNU_SOURCE
-
 #ifndef Py_BUILD_CORE_BUILTIN
 #    define Py_BUILD_CORE_MODULE 1
 #endif
 
 #include "Python.h"
-#include <internal/pycore_debug_offsets.h>  // _Py_DebugOffsets
-#include <internal/pycore_frame.h>          // FRAME_SUSPENDED_YIELD_FROM
-#include <internal/pycore_interpframe.h>    // FRAME_OWNED_BY_INTERPRETER
-#include <internal/pycore_llist.h>          // struct llist_node
-#include <internal/pycore_long.h>           // _PyLong_GetZero
-#include <internal/pycore_stackref.h>       // Py_TAG_BITS
+#include "internal/pycore_debug_offsets.h"  // _Py_DebugOffsets
+#include "internal/pycore_frame.h"          // FRAME_SUSPENDED_YIELD_FROM
+#include "internal/pycore_interpframe.h"    // FRAME_OWNED_BY_INTERPRETER
+#include "internal/pycore_llist.h"          // struct llist_node
+#include "internal/pycore_long.h"           // _PyLong_GetZero
+#include "internal/pycore_stackref.h"       // Py_TAG_BITS
 #include "../../Python/remote_debug.h"
 
 #include <assert.h>
@@ -40,10 +41,17 @@ extern "C" {
 #    define HAVE_PROCESS_VM_READV 0
 #endif
 
-#if defined(__APPLE__) && TARGET_OS_OSX
-#include <libproc.h>
-#include <sys/types.h>
-#define MAX_NATIVE_THREADS 4096
+#if defined(__APPLE__)
+#include <TargetConditionals.h>
+#  if !defined(TARGET_OS_OSX)
+     /* Older macOS SDKs do not define TARGET_OS_OSX */
+#    define TARGET_OS_OSX 1
+#  endif
+#  if TARGET_OS_OSX
+#    include <libproc.h>
+#    include <sys/types.h>
+#    define MAX_NATIVE_THREADS 4096
+#  endif
 #endif
 
 #ifdef MS_WINDOWS