librt_timers.m4: check if timer_create() is usable
librt_timers.m4: check if timer_create() is usable

Adds a custom check for the timer_create() function on the target
system.

It was found that on OpenBSD, the timer_create(), timer_settime(),
and timer_delete() functions were only stubbed out. They set
errno to ENOSYS and return -1. To detect this and replace the timer_*
functions with libcompat's, this m4 file will execute a test
program on the system, checking for the ENOSYS errno.

tarruda/check,tarruda/check,tarruda/check,tarruda/check,tarruda/check