niven build scripts updated

I finally managed to get some time to update the niven build scripts (possibly 50% rewritten ;) ). Now, it still supports Windows only, but it will support x64 and x86. The x64 support is currently held back until Boost 1.34 comes out. I’ve also used a conditional statement in Python for the first time (suffix = 'd' if conf == 'debug' else '') :)