We don't yet run on any system where pthread_t is an opaque type (and I bet
we never do). It's useful for the thread ID to match gdb's version, for now
just print out the pthread_t. Reference #1327.
os_thread.c:55:6: error: format '%p' expects argument of type 'void *',
but argument 5 has type 'pthread_t' [-Werror=format=]
os_thread.c:55:45: error: cast from function call of type 'pthread_t'
to non-matching type 'void *' [-Werror=bad-function-cast]
Merge pull request #1328 from wiredtiger/pthread-printf

Change how we print pthread IDs to match GDB on Linux.