configure: Cache result of pthread_condattr_setclock() check

configure: Cache result of pthread_condattr_setclock() check

Even if not using caching when running the configure script (-C) this
allows pre-defining the result by setting the environment variable
ss_cv_func_pthread_condattr_setclock_monotonic=yes|no|unknown
before/while running the script.

As the check requires running a test program this might be helpful
when cross-compiling to disable using monotonic time if
pthread_condattr_setclock() is defined but not actually usable with
CLOCK_MONOTONIC.

References #1502.