build directory for shadow build

QBS builds the code inside the directory I start it from. It would be nice if I can set a build directory somewhere else to keep my source directory clean.

For example:
Now (build inside source/qbs start location/contaminated source directory):

-- Project
    |
    └- src
        |
        └- qt-4-8-5-debug

Better (build outside source/manually set location/clean source directory):

-- Project
    |
    ├- src
    |
    └- build
        |
        └- qt-4-8-5-debug