Bug 1070442 - Support building gaia apps in parallel

@ochameau 

wow thanks:)

However, problem occurred in doing ```JSON.parse(appOptions)```. It just gave me ```SyntaxError: JSON.parse: unexpected character at line 1``` and after then, I try to dump the result of ```dump(JSON.stringify(appOptions))``` but in vain (It just showed ```JSON data``` but should be a something useful JSON string).

I need your help. I don't have any idea about how to debug / dump JSON in xpcshell. ;(