configure.ac: fix detection of clock_gettime library

configure.ac: fix detection of clock_gettime library

glibc before 2.17 requires link with librt for clock_gettime(). The
AC_SEARCH_LIBS check in configure.ac should detect this dependency.
Unfortunately commit cb77a25e51 (configure.ac: Remove obsolete AC_ERROR
and make formatting consistent) inadvertently renamed to clock_gettime2,
thus breaking librt detection.

Restore the correct clock_gettime() name.

Closes #439

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>