Support parallel debian builds; fix 6799
Support parallel debian builds; fix 6799

Adding '--parallel' to dh(1) calls permits parallel builds when
the user requests them, e.g. via the '-j' flag to dpkg-buildpackage
or other build helpers, or setting parallel=N in DEB_BUILD_OPTIONS
environment variable.

abn/geary,geekvigilante/geary,jimbonator/geary,martinra/geary,rschroll/geary,emersion/geary,glebihan/geary,glebihan/geary,jimbonator/geary,kpiwko/geary,rschroll/geary,aatishnn/geary,AtticHacker/geary,martinra/geary,tshikaboom/geary,mpdeimos/geary,emersion/geary,tshikaboom/geary,AtticHacker/geary,robol/geary,mjog/geary,aatishnn/geary,emersion/geary,mjog/geary,jimbonator/geary,AtticHacker/geary,robol/geary,robol/geary,githubnemo/geary,githubnemo/geary,aatishnn/geary,mjog/geary,githubnemo/geary,glebihan/geary,kyoushuu/geary,kyoushuu/geary,tshikaboom/geary,MohamedIM/geary-unofficial,martinra/geary