patch 8.1.1213: "make clean" in top dir does not cleanup indent test output

Problem:    "make clean" in top dir does not cleanup indent test output.
Solution:   Clean the indent test output.  Do not rely on the vim executable
            for that. (closes #4307)
diff --git a/Makefile b/Makefile
index 16fc84c..6ee3b1c 100644
--- a/Makefile
+++ b/Makefile
@@ -43,13 +43,18 @@
 	@if test "$@" = "test"; then \
 		$(MAKE) indenttest; \
 	fi
+	@# When the target is "clean" also clean for the indent tests.
+	@if test "$@" = "clean" -o "$@" = "distclean" -o "$@" = "testclean"; then \
+		cd runtime/indent && \
+			$(MAKE) clean; \
+	fi
 
 # Executable used for running the indent tests.
 VIM_FOR_INDENTTEST = ../../src/vim
 
 indenttest:
 	cd runtime/indent && \
-		$(MAKE) clean VIM="$(VIM_FOR_INDENTTEST)" && \
+		$(MAKE) clean && \
 		$(MAKE) test VIM="$(VIM_FOR_INDENTTEST)"