Make SIGEV_THREAD_ID decoding less glibc specific

Make SIGEV_THREAD_ID decoding less glibc specific

SIGEV_THREAD_ID decoding requires access to an internal member of
struct sigevent.  There seems to be no portable way to do it besides
adding a configure check.

* configure.ac (AC_CHECK_MEMBERS): Check for
struct sigevent._sigev_un._pad and struct sigevent.__pad.
* time.c (printsigevent): Use an appropriate struct sigevent member
to print thread id.

Reported by John Spencer.