Allow cross-compiling for Windows ARM64 (GH-343)

Also, use the setuptools build_ext command: this allows proper handling of cross-compilation added to setuptools but not to [deprecated] distutils.

buildlibxml.py

@@ -38,7 +38,9 @@ def download_and_extract_windows_binaries(destdir):
         if release_path in filename
     ]
 
-    if platform.machine() == 'ARM64':
+    # Check for native ARM64 build or the environment variable that is set by
+    # Visual Studio for cross-compilation (same variable as setuptools uses)
+    if platform.machine() == 'ARM64' or os.getenv('VSCMD_ARG_TGT_ARCH') == 'arm64':
         arch = "win-arm64"
     elif sys.maxsize > 2**32:
         arch = "win64"

setupinfo.py

@@ -3,9 +3,10 @@
 import os
 import os.path
 import subprocess
+
+from setuptools.command.build_ext import build_ext as _build_ext
 from distutils.core import Extension
 from distutils.errors import CompileError, DistutilsOptionError
-from distutils.command.build_ext import build_ext as _build_ext
 from versioninfo import get_base_dir
 
 try: