Set MACOSX_DEPLOYMENT_TARGET to 10.9, if not set by the user

cibuildwheel/macos.py

@@ -162,16 +162,17 @@ def build(project_dir, output_dir, test_command, before_test, test_requires, tes
             exit(1)
         call(['pip', 'install', '--upgrade', 'setuptools', 'wheel', 'delocate'], env=env)
 
-        # setup target platform, only required for python 3.5
+        # Set MACOSX_DEPLOYMENT_TARGET to 10.9, if the user didn't set it.
+        # CPython 3.5 defaults to 10.6, and pypy defaults to 10.3, causing
+        # warnings and potential problems if it's left unset.
+        env.setdefault('MACOSX_DEPLOYMENT_TARGET', '10.9')
+
         if config.version == '3.5':
-            if '_PYTHON_HOST_PLATFORM' not in env:
-                # cross-compilation platform override
-                env['_PYTHON_HOST_PLATFORM'] = 'macosx-10.9-x86_64'
-            if 'ARCHFLAGS' not in env:
-                # https://github.com/python/cpython/blob/a5ed2fe0eedefa1649aa93ee74a0bafc8e628a10/Lib/_osx_support.py#L260
-                env['ARCHFLAGS'] = '-arch x86_64'
-            if 'MACOSX_DEPLOYMENT_TARGET' not in env:
-                env['MACOSX_DEPLOYMENT_TARGET'] = '10.9'
+            # Cross-compilation platform override - CPython 3.5 has an
+            # i386/x86_64 version of Python, but we only want a x64_64 build
+            env.setdefault('_PYTHON_HOST_PLATFORM', 'macosx-10.9-x86_64')
+            # https://github.com/python/cpython/blob/a5ed2fe0eedefa1649aa93ee74a0bafc8e628a10/Lib/_osx_support.py#L260
+            env.setdefault('ARCHFLAGS', '-arch x86_64')
 
         # run the before_build command
         if before_build: