rewriter: fix warning about empty sources

mesonbuild/ast/introspection.py

@@ -278,6 +278,7 @@ class IntrospectionInterpreter(AstInterpreter):
         objects = []        # type: T.List[T.Any]
         empty_sources = []  # type: T.List[T.Any]
         # Passing the unresolved sources list causes errors
+        kwargs_reduced['_allow_no_sources'] = True
         target = targetclass(name, self.subdir, self.subproject, for_machine, empty_sources, [], objects,
                              self.environment, self.coredata.compilers[for_machine], kwargs_reduced)
         target.process_compilers()

mesonbuild/build.py

@@ -743,11 +743,12 @@ class BuildTarget(Target):
         # 2. Compiled objects created by and extracted from another target
         self.process_objectlist(objects)
         self.process_kwargs(kwargs)
-        self.check_unknown_kwargs(kwargs)
-        if not any([self.sources, self.generated, self.objects, self.link_whole_targets, self.structured_sources]):
+        if not any([self.sources, self.generated, self.objects, self.link_whole_targets, self.structured_sources,
+                    kwargs.pop('_allow_no_sources', False)]):
             mlog.warning(f'Build target {name} has no sources. '
                          'This was never supposed to be allowed but did because of a bug, '
                          'support will be removed in a future release of Meson')
+        self.check_unknown_kwargs(kwargs)
         self.validate_install()
         self.check_module_linking()