Better detection for universal2. (#358)

Previously, we assumed that only "universal2" builds of Python would want to build extensions as "universal2". But apparently the "arm64" builds of Python that come on M1 machines also want to build extensions as "universal2".

setup.py

@@ -37,6 +37,15 @@ def copy_tree(src, dst):
         shutil.copytree(src, dst)
 
 
+def is_building_macos_universal2():
+    """Return whether extension will be built as an Apple universal binary (works on both x86_64 and arm64)"""
+    if not sys.platform == 'darwin':
+        return False
+
+    cflags = sysconfig.get_config_var('CFLAGS')
+    return '-arch x86_64' in cflags and '-arch x86_64' in cflags
+
+
 def determine_cross_compile_args():
     host_arch = platform.machine()
     if (host_arch == 'AMD64' or host_arch == 'x86_64') and is_32bit() and sys.platform != 'win32':
@@ -205,10 +214,14 @@ def _build_dependency_impl(self, aws_lib, build_dir, install_path, osx_arch=None
         run_cmd(build_cmd)
 
     def _build_dependency(self, aws_lib, build_dir, install_path):
-        if sys.platform == 'darwin' and self.plat_name.endswith('universal2'):
+        if is_building_macos_universal2():
             # create macOS universal binary by compiling for x86_64 and arm64,
             # each in its own subfolder, and then creating a universal binary
             # by gluing the two together using `lipo`.
+            #
+            # The AWS C libs don't support building for multiple architectures
+            # simultaneously (too much confusion at cmake configure time).
+            # So we build each architecture one at a time.
 
             # x86_64
             self._build_dependency_impl(