Enable thread support in the MS-Windows build.

Enable thread support in the MS-Windows build.

 src/systhread.h (w32thread_critsect, w32thread_cond_t, sys_mutex_t)
 (sys_cond_t, sys_thread_t) [WINDOWSNT]: New data types.
 src/systhread.c (sys_mutex_init, sys_mutex_lock, sys_mutex_unlock)
 (sys_mutex_destroy, sys_cond_init, sys_cond_wait)
 (sys_cond_signal, sys_cond_broadcast, sys_cond_destroy)
 (sys_thread_self, sys_thread_equal, w32_beginthread_wrapper)
 (sys_thread_create, sys_thread_yield) [WINDOWSNT]: New functions.

 configure.ac (THREADS_ENABLED): Enable threads for MinGW, even
 if pthreads is not available.