port: advance MSVC600 Python core frontier

scripts/vc6-probe-pythoncore-frontier.sh

@@ -127,6 +127,10 @@ def walk(node, inherited_condition=""):
         for child in node:
             if child.tag == ns + "PreprocessorDefinitions":
                 defs.extend(split_definitions(child.text or ""))
+        if rel == "cpython/Modules/binascii.c":
+            # binascii.c has a complete local CRC32 fallback.  Do not require
+            # the external zlib headers in the minimal VC6 frontier.
+            defs = [define for define in defs if define != "USE_ZLIB_CRC32"]
         action = classify(rel, inherited_condition, condition)
         reason = action
         if action == "compile" and norm_error: