Enable build using toolchain without pthreads

* src/atomic_ops.c: Include pthread.h only if not AO_NO_PTHREADS (new
macro).
* src/atomic_ops.c (AO_pt_lock): Define only unless AO_NO_PTHREADS.
* tests/test_atomic.c (main): Skip test_atomic_pthreads if both
AO_NO_PTHREADS and AO_USE_PTHREAD_DEFS are defined (preventing test
link failure if AO_NO_PTHREADS passed via CFLAGS).
Update AUTHORS
Better document configure THREADDLLIBS variable

* configure.ac (THREADDLLIBS, have_pthreads): Add comments.
Define AO_NO_PTHREADS in configure if no pthreads (Win32 and VxWorks)

* configure.ac (have_pthreads): Do not define to true if check for
pthread_self failed.
* configure.ac (AO_NO_PTHREADS): Define if not have_pthreads.
Add non-generalized double-CAS-full implementation for AArch64

* src/atomic_ops/sysdeps/gcc/aarch64.h
(AO_double_compare_and_swap_full): New function.
Add comment about store_full implementation for x86

* src/atomic_ops/sysdeps/gcc/x86.h: Add comment about AO_store_full
generalization.