Renamed --libstatedir to --statedir.

In BSD systems, it's usually </var/db> or some other dir under </var>
that is not </var/lib>, so $statedir is a more generic name.  See
hier(7).

Reported-by: Andrei Zeliankou <zelenkov@nginx.com>
Reported-by: Zhidao Hong <z.hong@f5.com>
Reviewed-by: Konstantin Pavlov <thresh@nginx.com>
Reviewed-by: Andrew Clayton <a.clayton@nginx.com>
Cc: Liam Crilly <liam@nginx.com>
Signed-off-by: Alejandro Colomar <alx@nginx.com>

configure

@@ -72,7 +72,7 @@ cat << END >> $NXT_AUTO_CONFIG_H
 #define NXT_PID                "$NXT_PID"
 #define NXT_LOG                "$NXT_LOG"
 #define NXT_MODULESDIR         "$NXT_MODULESDIR"
-#define NXT_LIBSTATEDIR        "$NXT_LIBSTATEDIR"
+#define NXT_STATEDIR           "$NXT_STATEDIR"
 #define NXT_TMPDIR             "$NXT_TMPDIR"
 
 #define NXT_CONTROL_SOCK       "$NXT_CONTROL"