Use AC_DEFINE for USE_THREAD_DB

Use AC_DEFINE for USE_THREAD_DB instead of manually passing it down
through -D flags.

gdb/gdbserver/
2013-04-17  Pedro Alves  <palves@redhat.com>

	* configure.ac (USE_THREAD_DB): Delete variable.
	(if test "$srv_linux_thread_db" = "yes"): AC_DEFINE USE_THREAD_DB.
	Don't AC_SUBST USE_THREAD_DB.
	* Makefile.in (INTERNAL_CFLAGS): Remove @USE_THREAD_DB@.
	* config.in, configure: Regenerate.