gdb: Drop use of obsolete AC_TYPE_SIGNAL

gdb: Drop use of obsolete AC_TYPE_SIGNAL

Since we're using sighandler_t, nothing else refers to RETSIGTYPE in
gdb.

(Actually, given gdb/remote.c has been assuming signal handlers return
void for a long time, we could have gotten get rid of this even
without gnulib's sighandler_t.)

gdb/ChangeLog:
2015-08-27  Pedro Alves  <palves@redhat.com>

	* configure.ac: Remove AC_TYPE_SIGNAL call.
	* configure, config.in: Regenerate.