add check-valgrind target

Original commit message from CVS:
add check-valgrind target
diff --git a/Makefile.am b/Makefile.am
index 558e95e..cfe228d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -33,3 +33,6 @@
 
 include $(top_srcdir)/common/release.mak
 # include $(top_srcdir)/common/po.mak
+
+check-valgrind:
+	@true