autotools: include pthread.h when checking for clockid_t/timer_t
autotools: include pthread.h when checking for clockid_t/timer_t

Must include pthread.h to check for clockid_t and timer_t. This is
because MinGW and MinGW-w64 both put the time related definitions
in pthread headers and not in sys/types.h. Without optionally
including pthread.h, the check for these types may mistakenly fail.

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