[build] Add NETBSD leveldb target to configure.ac

configure.ac

@@ -449,6 +449,9 @@ case $host in
    *openbsd*)
      LEVELDB_TARGET_FLAGS="-DOS_OPENBSD"
      ;;
+   *netbsd*)
+     LEVELDB_TARGET_FLAGS="-DOS_NETBSD"
+     ;;
    *)
      OTHER_OS=`echo ${host_os} | awk '{print toupper($0)}'`
      AC_MSG_WARN([Guessing LevelDB OS as OS_${OTHER_OS}, please check whether this is correct, if not add an entry to configure.ac.])