Check if clock_gettime() needs librt
Check if clock_gettime() needs librt

According to the manuals, clock_gettime() requires linking with -lrt
for glibc versions before 2.17.

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