[build] Fix network time plugin build issues

Link clock functions explicitly to the POSIX realtime library when using glibc version < 2.17 (otherwise, some compilation environments implicitly link libpthread, others fail). Also, there was a typo that broke the '--disable-nettime' configure option.

Signed-off-by: Martti Piirainen <martti.piirainen@oss.tieto.com>