Comment 3 for bug 1609043

From log:

Setting up mysql-server-5.7 (5.7.13-0ubuntu0.16.04.2) ...
Installing new version of config file /etc/apparmor.d/usr.sbin.mysqld ...
Installing new version of config file /etc/init.d/mysql ...
Installing new version of config file /etc/init/mysql.conf ...
Installing new version of config file /etc/logrotate.d/mysql-server ...
Installing new version of config file /etc/mysql/debian-start ...
/usr/lib/x86_64-linux-gnu/libproxy.so.1: symbol _ZTTNSt7__cxx1119basic_ostringstreamIcSt11char_traitsIcESaIcEEE, version GLIBCXX_3.4.21 not defined in file libstdc++.so.6 with link time reference
Failed to load module: /usr/lib/x86_64-linux-gnu/gio/modules/libgiolibproxy.so
update-alternatives: using /etc/mysql/mysql.cnf to provide /etc/mysql/my.cnf (my.cnf) in auto mode
Renaming removed key_buffer and myisam-recover options (if present)
ERROR: Unable to start MySQL server:
mysqld: relocation error: mysqld: symbol _ZTVNSt7__cxx1115basic_stringbufIcSt11char_traitsIcESaIcEEE, version GLIBCXX_3.4.21 not defined in file libstdc++.so.6 with link time reference
Please take a look at https://wiki.debian.org/Teams/MySQL/FAQ for tips on fixing common upgrade issues.
Once the problem is resolved, run apt-get --fix-broken install to retry.
dpkg: error processing package mysql-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 127