stabilize iteration order for dictionaries

The order of keys in dictionaries cannot be relied upon, because the hash
values are randomized by Python.  Whenever we iterate on dictionaries and
meson.build generates a list during the iteration, the different iteration
orders may cause random changes in the command line and cause ninja to
rebuild a lot of files unnecessarily.

mesonbuild/interpreterbase.py

@@ -823,7 +823,7 @@ The result of this is undefined and will become a hard error in a future Meson r
         elif isinstance(items, dict):
             if len(node.varnames) != 2:
                 raise InvalidArguments('Foreach on dict unpacks key and value')
-            for key, value in items.items():
+            for key, value in sorted(items.items()):
                 self.set_variable(node.varnames[0], key)
                 self.set_variable(node.varnames[1], value)
                 try:
@@ -1166,7 +1166,7 @@ The result of this is undefined and will become a hard error in a future Meson r
         if method_name == 'keys':
             if len(posargs) != 0:
                 raise InterpreterException('keys() takes no arguments.')
-            return list(obj.keys())
+            return sorted(obj.keys())
 
         raise InterpreterException('Dictionaries do not have a method called "%s".' % method_name)