patch 9.0.1630: "make clean" at the toplevel fails

Problem:    "make clean" at the toplevel fails.
Solution:   Clean the indent and syntax directories in a sub-shell.  (Ben
            Jackson, closes #12536, closes #12526)
diff --git a/Makefile b/Makefile
index f70fb85..2448105 100644
--- a/Makefile
+++ b/Makefile
@@ -46,10 +46,8 @@
 	fi
 	@# When the target is "clean" also clean for the indent and syntax tests.
 	@if test "$@" = "clean" -o "$@" = "distclean" -o "$@" = "testclean"; then \
-		cd runtime/indent && \
-			$(MAKE) clean; \
-		cd runtime/syntax && \
-			$(MAKE) clean; \
+		(cd runtime/indent && $(MAKE) clean); \
+		(cd runtime/syntax && $(MAKE) clean); \
 	fi
 
 # Executable used for running the indent tests.