Fix check_c_source_compiles with pthread.

Also fix the variables: we need to check for PTHREAD_PRIO_INHERIT
and PTHREAD_CREATE_JOINABLE (not PTHREAD_CREATE_UNDETACHED) and
internally use HAVE_PTHREAD_PRIO_INHERIT and PTHREAD_CREATE_JOINABLE
(and not HAVE_PTHREAD_CREATE_JOINABLE).
cmake/config.h.in actually had the right variables.

BUG=webp:498

Change-Id: Ibf6cf854337cea5781a74316024f8ff4960366d7
(cherry picked from commit ceddb5fc8d077fb1d5d4536012c33172645c69cf)

• cmake/config.h.in[diff]
• cmake/deps.cmake[diff]