Allow sdktool to run in an elevated context

This fixes an issue with installation on Mac: We run sdktool with
'elevated' rights if we e.g. install in a global directory, forcing
the effective user id to be 0. However, since Qt 5.3.0 QCoreApplication
aborts if geteuid() != getuid().

Change-Id: I678f405fc0cb31dbf9c0487f674bb8dbe5e2e95e
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>