/usr/lib/x86_64-linux-gnu/libmpi_cxx.so is in libopenmpi-dev 2.1.1-8.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libmpi_cxx.so.20
/usr/lib/x86_64-linux-gnu/libmpi_cxx.so.20