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.

git-svn-id: svn+ssh://svn.code.sf.net/p/check/code/trunk@981 64e312b2-a51f-0410-8e61-82d0ca0eb02a