configure.ac: check for clock_gettime LIBS

In commit 4e77e09621795a7a98d9293dbed0a082d603f10c clock_gettime() usage
was introduced, however there's no check for proper support libraries
for this function, and for glibc < 2.17 it needs librt.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>