Revise mythread.h.

Revise mythread.h.

This adds:

  - mythread_sync() macro to create synchronized blocks

  - mythread_cond structure and related functions
    and macros for condition variables with timed
    waiting using a relative timeout

  - mythread_create() to create a thread with all
    signals blocked

Some of these wouldn't need to be inline functions,
but I'll keep them this way for now for simplicity.

For timed waiting on a condition variable, librt is
now required on some systems to use clock_gettime().
configure.ac was updated to handle this.