build: add check for clock_gettime() in librt

zuctest is another clock_gettime() user that fails to link against librt when
necessary.

Instead of adding another -lrt LDADD entry i've opted for the saner way and
converted the check to a configure test that will set CLOCK_GETTIME_LIBS
appropiately and replaced all instances of -lrt with it.
Built-tested against old and new glibc.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
2 files changed