Summary: Make the build not run in parallel (fixes: #991133)

Summary: Make the build not run in parallel (fixes: #991133)

The changes from Helmut to make cross-building work moved us from
non-parallel to parallel builds. Unfortunately, the build system isn't
parallel-build-safe (see, for example, the calls of make from within
the Makefile), and this meant that in some circumstances the
concordance file ended up empty.

This package is very quick to build, so tell dh_auto_build to not
attempt parallel building.