radvd: fix build against older glibc

clock_gettime() requires -lrt when linking against glibc older than 2.17.

Fixes:
http://autobuild.buildroot.net/results/e07/e07dd9a82ae4c8778b6901b9bd777f5affb9293a/

[Peter: add comment in .mk as well]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/radvd/radvd.mk

@@ -8,6 +8,8 @@ RADVD_VERSION = 1.13
 RADVD_SOURCE = radvd-$(RADVD_VERSION).tar.xz
 RADVD_SITE = http://www.litech.org/radvd/dist
 RADVD_DEPENDENCIES = host-bison flex libdaemon host-flex host-pkgconf
+# clock_gettime() requires -lrt when linking against glibc older than 2.17
+RADVD_CONF_ENV = LIBS=-lrt
 
 define RADVD_INSTALL_INITSCRIPT
 	$(INSTALL) -m 0755 package/radvd/S50radvd $(TARGET_DIR)/etc/init.d