diff --git a/c/src/optman/preinstall.am b/c/src/optman/preinstall.am
index 41d10b7f29..77e48975ec 100644
--- a/c/src/optman/preinstall.am
+++ b/c/src/optman/preinstall.am
@@ -6,10 +6,10 @@ $(srcdir)/preinstall.am: Makefile.am
 endif
 
 PREINSTALL_DIRS =
-DISTCLEANFILES += $(PREINSTALL_DIRS)
+DISTCLEANFILES = $(PREINSTALL_DIRS)
 
 TMPINSTALL_FILES =
-CLEANFILES += $(TMPINSTALL_FILES)
+CLEANFILES = $(TMPINSTALL_FILES)
 
 $(PROJECT_LIB)/$(dirstamp):
 	@$(mkdir_p) $(PROJECT_LIB)