timer-time: link explicitly with pthreads on glibc

timer-time: link explicitly with pthreads on glibc

To support static linking we need to explicitly
specify the pthreads library which is referenced
(but not used on modern Linux kernels at least).

* m4/timer_time.m4 (LIB_TIMER_TIME): Add -lpthread
when newer GLIBC is detected as that contains
the pthread emulation.
* modules/timer-time: Depend on threadlib to
pull in the appropriate library to link