Prepare automatically setting Py_LIMITED_API (once it's ready).

Cython/Compiler/ModuleNode.py

@@ -665,7 +665,9 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             code.putln(json.dumps(metadata, indent=4, sort_keys=True))
             code.putln("END: Cython Metadata */")
             code.putln("")
+
         code.putln("#define PY_SSIZE_T_CLEAN")
+        self._put_setup_code(code, "InitLimitedAPI")
 
         for inc in sorted(env.c_includes.values(), key=IncludeCode.sortkey):
             if inc.location == inc.INITIAL:

Cython/Utility/ModuleSetupCode.c

+/////////////// InitLimitedAPI ///////////////
+
+#if defined(CYTHON_LIMITED_API) && 0  /* disabled: enabling Py_LIMITED_API needs more work */
+  #ifndef Py_LIMITED_API
+    #if CYTHON_LIMITED_API+0 > 0
+      #define Py_LIMITED_API CYTHON_LIMITED_API
+    #else
+      #define Py_LIMITED_API 0x03030000
+    #endif
+  #endif
+#endif
+
+
 /////////////// CModulePreamble ///////////////
 
 #include <stddef.h> /* For offsetof */