Avoid a race condition when making directories
Avoid a race condition when making directories

This can happen when doing a concurrent build with -jX.

duncanmortimer/ledger,paulbdavis/ledger,jwakely/ledger,paulbdavis/ledger,ledger/ledger,jwakely/ledger,duncanmortimer/ledger,duncanmortimer/ledger,afh/ledger,jwakely/ledger,afh/ledger,jwakely/ledger,afh/ledger,ledger/ledger,jwakely/ledger,paulbdavis/ledger,afh/ledger,jwakely/ledger,ledger/ledger,afh/ledger,paulbdavis/ledger,ledger/ledger,paulbdavis/ledger,ledger/ledger,duncanmortimer/ledger,duncanmortimer/ledger,jwakely/ledger,duncanmortimer/ledger,paulbdavis/ledger,duncanmortimer/ledger,paulbdavis/ledger