-# List all subdirectories here. Each contains its own Makefile.local
-subdirs := compat completion emacs lib man parse-time-string
-subdirs += performance-test util test
+# List all subdirectories here. Each contains its own Makefile.local.
+# Use of '=', without '+=', seems to be required for out-of-tree
+# builds to work.
+subdirs = compat completion emacs lib man parse-time-string performance-test util test