build.sh: set LDFLAGS=-lrt for glibc < 2.17 in lwip build
build.sh: set LDFLAGS=-lrt for glibc < 2.17 in lwip build

The link step for the native lwip build fails on platforms that
use a glibc version prior 2.17. The reason is that librt is needed
for clock_gettime() in that environment. configure detects this
correctly but the lwip example comes with a simple hand-craftet
Makefile that does not use the configure results.

This quick hack checks for the output of the check for clock_gettime()
and includes the required libraries in order to fix the travis build.

gebart/libcoap,authmillenon/libcoap,gebart/libcoap,authmillenon/libcoap,authmillenon/libcoap,gebart/libcoap