First time run of make.js leads to error "file already exists"

[ryandle]

This repros consistently for me (node 0.10.25) if you delete the build folder and run the following:

C:\Repos\Glimpse.Client>node make.js

fs.js:642
  return binding.mkdir(pathModule._makeLong(path),
                 ^
Error: EEXIST, file already exists 'C:\Repos\Glimpse.Client\build'
    at Object.fs.mkdirSync (fs.js:642:18)
    at C:\Repos\Glimpse.Client\make.js:46:24
    at Object.cb [as oncomplete] (fs.js:168:19)

Looks like a race in the use of fs.exists() && multiple calls to output() with the same value for outputDir.