BLD: fix error when config variable MACOSX_DEPLOYMENT_TARGET is not s…

…et, such as with PyPy (pandas-dev#26536)

setup.py

@@ -442,19 +442,18 @@ def run(self):
     if debugging_symbols_requested:
         extra_compile_args.append('-g')
 
-# For mac, ensure extensions are built for macos 10.9 when compiling on a
-# 10.9 system or above, overriding distuitls behaviour which is to target
+# For mac, ensure extensions are built for at least macOS 10.9 when compiling on a
+# 10.9 system or above, overriding CPython distuitls behaviour which is to target
 # the version that python was built for. This may be overridden by setting
 # MACOSX_DEPLOYMENT_TARGET before calling setup.py
 if is_platform_mac():
     if 'MACOSX_DEPLOYMENT_TARGET' not in os.environ:
         current_system = LooseVersion(platform.mac_ver()[0])
         python_target = LooseVersion(
-            get_config_var('MACOSX_DEPLOYMENT_TARGET'))
+            get_config_vars().get('MACOSX_DEPLOYMENT_TARGET', current_system))
         if python_target < '10.9' and current_system >= '10.9':
             os.environ['MACOSX_DEPLOYMENT_TARGET'] = '10.9'
 
-
 # enable coverage by building cython files by setting the environment variable
 # "PANDAS_CYTHON_COVERAGE" (with a Truthy value) or by running build_ext
 # with `--with-cython-coverage`enabled