Only #define PY_SSIZE_T_CLEAN if it is not already #defined

Signed-off-by: Andrew V. Jones <andrew.jones@vector.com>

Cython/Compiler/ModuleNode.py

@@ -634,7 +634,10 @@ 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("#ifndef PY_SSIZE_T_CLEAN")
         code.putln("#define PY_SSIZE_T_CLEAN")
+        code.putln("#endif /* PY_SSIZE_T_CLEAN */")
 
         for inc in sorted(env.c_includes.values(), key=IncludeCode.sortkey):
             if inc.location == inc.INITIAL: