envconfig: mingw is also Windows

Also change the same idiom for Darwin to use a set().

mesonbuild/envconfig.py

@@ -198,7 +198,7 @@ class MachineInfo:
         """
         Machine is windows?
         """
-        return self.system == 'windows'
+        return self.system in {'windows', 'mingw'}
 
     def is_cygwin(self) -> bool:
         """
@@ -216,7 +216,7 @@ class MachineInfo:
         """
         Machine is Darwin (iOS/OS X)?
         """
-        return self.system in ('darwin', 'ios')
+        return self.system in {'darwin', 'ios'}
 
     def is_android(self) -> bool:
         """