set WIN_VER in compiler configuration

we don't have source control over libzmq itself to add includes

buildutils/config.py

@@ -157,9 +157,21 @@ def discover_settings(conf_base=None):
         'bundle_msvcp': None,
         'build_ext': {},
         'bdist_egg': {},
+        'win_ver': None,
     }
     if sys.platform.startswith('win'):
         settings['have_sys_un_h'] = False
+        # target Windows version, sets WIN_VER, _WIN32_WINNT macros
+        # see https://docs.microsoft.com/en-us/cpp/porting/modifying-winver-and-win32-winnt for reference
+        # see https://github.com/python/cpython/blob/v3.9.1/PC/pyconfig.h#L137-L159
+        # for CPython's own values
+        if sys.version_info >= (3, 9):
+            # CPython 3.9 targets Windows 8 (0x0602)
+            settings["win_ver"]: "0x0602"
+        else:
+            # older Python, target Windows 7 (0x0601)
+            # CPython itself targets Vista (0x0600)
+            settings["win_ver"]: "0x0601"
 
     if conf_base:
         # lowest priority

setup.py

@@ -411,6 +411,13 @@ def init_settings_from_config(self):
         if cfg['have_sys_un_h']:
             settings['define_macros'].append(('HAVE_SYS_UN_H', 1))
 
+        if cfg['win_ver']:
+            # set target minimum Windows version
+            settings['define_macros'].extend([
+                ('WIN_VER', cfg['win_ver']),
+                ('_WIN32_WINNT', cfg['win_ver']),
+            ])
+
         if cfg.get('zmq_draft_api'):
             settings['define_macros'].append(('ZMQ_BUILD_DRAFT_API', 1))