The automatically generated hostname is too long: OpenMPI uses it as
part of a path to UNIX sockets, so there is a length limitation.

Debian stretch now has DUNE 2.5.

Currently build from the Git branch, but might switch to using packages
later.

Before the unset `DUNECI_OPTS` was used in the next line...

Otherwise no opts file is used at all and the code is built without any
optimization, leading to timeouts in tests.