Fix unpickling of dynamic subclasses of ConfigToolDependency

This code isn't used as present: the actual objects we use and which get
pickled are either static subclasses of ConfigToolDependency, or special
purpos subclasses of Dependency which get attributes copied from an instance
of a dynamic subclass of ConfigToolDependency which is then discarded.

This is fortunate, as it doesn't work: the pickled reduction is a call to
the dynamic subclass's constructor, but the superclass constructors rely on
the environment object being fully initialized, which hasn't happened yet
during unpickling.

Avoid this by having a pickled reduction which is just a call to create the
dynamic subclass object, and relying on the default __setstate__ behavior to
unpickle the object's __dict__.

mesonbuild/dependencies/base.py

@@ -236,14 +236,17 @@ class ConfigToolDependency(ExternalDependency):
         # instantiated and returned. The reduce function (method) is also
         # attached, since python's pickle module won't be able to do anything
         # with this dynamically generated class otherwise.
-        def reduce(_):
-            return (cls.factory,
-                    (name, environment, language, kwargs, tools, tool_name))
+        def reduce(self):
+            return (cls._unpickle, (), self.__dict__)
         sub = type('{}Dependency'.format(name.capitalize()), (cls, ),
                    {'tools': tools, 'tool_name': tool_name, '__reduce__': reduce})
 
         return sub(name, environment, language, kwargs)
 
+    @classmethod
+    def _unpickle(cls):
+        return cls.__new__(cls)
+
     def find_config(self, versions=None):
         """Helper method that searchs for config tool binaries in PATH and
         returns the one that best matches the given version requirements.