QBS should not install to sysroot by default

Because of the way we set up profiles on OS X through Xcode now, all profiles have a sysroot set.

By default, QBS tries to install to here. This is wrong, and will thankfully fail (permissions).

Users should NOT have to manually set --install-root on the QBS command line. This behavior is clearly undesirable and we need a better default or methodology to handle this use case.