Allow custom_target do depend on indexed output of custom_target

Fixes: #3494

mesonbuild/backend/backends.py

@@ -762,7 +762,7 @@ class Backend:
                 fname = [os.path.join(self.build_to_src, target.subdir, i)]
             elif isinstance(i, build.BuildTarget):
                 fname = [self.get_target_filename(i)]
-            elif isinstance(i, build.CustomTarget):
+            elif isinstance(i, (build.CustomTarget, build.CustomTargetIndex)):
                 fname = [os.path.join(self.get_target_dir(i), p) for p in i.get_outputs()]
             elif isinstance(i, build.GeneratedList):
                 fname = [os.path.join(self.get_target_private_dir(target), p) for p in i.get_outputs()]

mesonbuild/build.py

@@ -1974,7 +1974,7 @@ def get_sources_string_names(sources):
             s = s.held_object
         if isinstance(s, str):
             names.append(s)
-        elif isinstance(s, (BuildTarget, CustomTarget, GeneratedList)):
+        elif isinstance(s, (BuildTarget, CustomTarget, CustomTargetIndex, GeneratedList)):
             names += s.get_outputs()
         elif isinstance(s, File):
             names.append(s.fname)