Fix build error on MySQL 5.6.36 and MySQL 5.7.18

The member of Fileld::part_of_key_not_clustered is removed.

  .Tpo -c ha_mroonga.cpp  -fPIC -DPIC -o .libs/ha_mroonga_la-ha_mroonga.o
  ==> centos-6-x86_64: ha_mroonga.cpp: In member function 'void ha_mroonga::wrapper_overwrite_index_bits()':
  ==> centos-6-x86_64: ha_mroonga.cpp:4290: error: 'class Field' has no member named 'part_of_key_not_clustered'
  ==> centos-6-x86_64: ha_mroonga.cpp:4306: error: 'class Field' has no member named 'part_of_key_not_clustered'
  ==> centos-6-x86_64: make[2]: *** [ha_mroonga_la-ha_mroonga.lo] Error 1
  ==> centos-6-x86_64: make[2]: *** Waiting for unfinished jobs....
  ==> centos-6-x86_64: mv -f .deps/ha_mroonga_la-mrn_table.Tpo .

ha_mroonga.cpp

@@ -4287,7 +4287,9 @@ void ha_mroonga::wrapper_overwrite_index_bits()
   {
     Field *field = table_share->field[i];
     field->part_of_key.clear_all();
+#ifdef MRN_HAVE_MYSQL_FIELD_PART_OF_KEY_NOT_CLUSTERED
     field->part_of_key_not_clustered.clear_all();
+#endif
     field->part_of_sortkey.clear_all();
   }
   for (i = 0; i < table_share->keys; i++) {
@@ -4303,7 +4305,9 @@ void ha_mroonga::wrapper_overwrite_index_bits()
         {
           table_share->keys_for_keyread.set_bit(i);
           field->part_of_key.set_bit(i);
+#ifdef MRN_HAVE_MYSQL_FIELD_PART_OF_KEY_NOT_CLUSTERED
           field->part_of_key_not_clustered.set_bit(i);
+#endif
         }
         if (index_flags(i, j, 1) & HA_READ_ORDER)
           field->part_of_sortkey.set_bit(i);

mrn_mysql_compat.h

@@ -351,4 +351,9 @@
   (c == FN_LIBCHAR || c == FN_LIBCHAR2)
 #endif
 
+#if ((MYSQL_VERSION_ID < 50636) || \
+    (MYSQL_VERSION_ID >= 50700 && MYSQL_VERSION_ID < 50718)) && !defined(MRN_MARIADB_P)
+#  define MRN_HAVE_MYSQL_FIELD_PART_OF_KEY_NOT_CLUSTERED
+#endif
+
 #endif /* MRN_MYSQL_COMPAT_H_ */