avoid pointless overhead for DESTDIR builds

http://bugzilla.netfilter.org/show_bug.cgi?id=560

--- Makefile.in
+++ Makefile.in
@@ -1530,7 +1530,7 @@
 # ldconfig may fail when we are not root (as is the case in build systems)
 # so add appropriate protection that it does not let `make` fail.
 install-exec-hook:
-	-/sbin/ldconfig || :;
+	-[ -z "$(DESTDIR)" ] && /sbin/ldconfig
 # Tell versions [3.59,3.63) of GNU make to not export all variables.
 # Otherwise a system limit (for SysV at least) may be exceeded.
 .NOEXPORT: