Set names of worker threads
    
    This adds some configury so that gdb can set the names of worker
    threads.  This makes them show up more nicely when debugging gdb
    itself.
    
    2019-10-19  Tom Tromey  <tom@tromey.com>
    
    	* gdbsupport/thread-pool.c (thread_pool::set_thread_count): Set
    	name of worker thread.
    	* gdbsupport/common.m4 (GDB_AC_COMMON): Check for
    	pthread_setname_np.
    	* configure, config.in: Rebuild.
    
    gdb/gdbserver/ChangeLog
    2019-10-19  Tom Tromey  <tom@tromey.com>
    
    	* configure, config.in: Rebuild.
    
    Change-Id: I60473d65ae9ae14d8c56ddde39684240c16aaf35