wrapper: use CMAKE_SYSTEM_NAME to detect Windows

wrapper.py.in

@@ -76,16 +76,16 @@ defaults = {
 env = defaults.copy()
 env.update(os.environ)
 
-# MinGW pretends to be its own platform. Try to work around their deception by
-# testing if the platform string looks like something that's actually useful.
-def is_really_windows() -> bool:
-    name = platform.system()
+def host_platform(tuple: str) -> str:
+    return {
+        'x86_64-w64-mingw32': 'Windows'
+    }[tuple]
 
-    if name == 'Windows':
-        return True
-    if name.startswith('MINGW'):
-        return True
-    return False
+
+# Don't use `platform.system()` as MinGW will pretend to be its own platform,
+# and cross compiling will give you CBUILD rather than CHOST.
+def is_really_windows() -> bool:
+    return "@CMAKE_SYSTEM_NAME@" == "Windows"
 
 
 # MSYS2 wants to fuck us over by supplying a CMake that gives something