Make Cargo aware of how many jobs Make can run
Make Cargo aware of how many jobs Make can run

All the credit goes to Anatoly Sablin (@ma1uta): he's the one who told
me about Make jobserver and its support in Cargo. I'm merely applying
that knowledge to my particular problem.

Fixes #768.

der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat