Mark release target as virtual

Change-Id: Iee747faa3171663f1874a5eacddd56607de55297
diff --git a/Makefile.am b/Makefile.am
index 5cf1a45..da08bf3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -39,6 +39,8 @@
 	COPYING \
 	README
 
+.PHONY: release
+
 @RELMAKE@
 
 dox: FORCE