Fix __thread configure check on Clang

AC_TRY_COMPILE puts the code in a function already, and Clang didn't like
the function within the function declaration. This lead to test failure.

Clang now properly detects __thread support.
Replace ringbuffer in Packet Pool with a stack for better cache locality

Using a stack for free Packet storage causes recently freed Packets to be
reused quickly, while there is more likelihood of the data still being in
cache.

The new structure has a per-thread private stack for allocating Packets
which does not need any locking. Since Packets can be freed by any thread,
there is a second stack (return stack) for freeing packets by other threads.
The return stack is protected by a mutex. Packets are moved from the return
stack to the private stack when the private stack is empty.

Returning packets back to their "home" stack keeps the stacks from getting out
of balance.

The PacketPoolInit() function is now called by each thread that will be
allocating packets. Each thread allocates max_pending_packets, which is a
change from before, where that was the total number of packets across all
threads.
For PktPool add local pending freed packets list.

Better handle the autofp case where one thread allocates the majority
of the packets and other threads free those packets.

Add a list of locally pending packets. The first packet freed goes on the
pending list, then subsequent freed packets for the same Packet Pool are
added to this list until it hits a fixed number of packets, then the
entire list of packets is pushed onto the pool's return stack. If a freed
packet is not for the pending pool, it is freed immediately to its pool's
return stack, as before.

For the autofp case, since there is only one Packet Pool doing all the
allocation, every other thread will keep a list of pending packets for
that pool.

For the worker run mode, most packets are allocated and freed locally. For
the case where packets are being returned to a remote pool, a pending list
will be kept for one of those other threads, all others are returned as before.

Which remote pool for which to keep a pending list is changed each time the
pending list is returned. Since the return pending pool is cleared when it is
freed, then next packet to be freed chooses the new pending pool.
Implement thread specific data option when __thread is not available.
Use posix_memalign instead of mm_malloc on non-Windows systems.
Add error checking for pthread_setspecific() and pthread_key_create().