Set names of worker threads

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