Fix failure to build from source with modern _host_ glibc/gcc.

One should not define __USE_GNU directly, and instead _GNU_SOURCE
should be defined as early as possible, before first (indirect)
inclusion of features.h

Change-Id: I4d109d4fa0c3d266b873d2a0a66a52e1a4faa234
Signed-off-by: Dmitrijs Ledkovs <dmitrijs.ledkovs@canonical.com>
toolbox: Add levelFromUid categorization support to restorecon

This is necessary for the app data relabeling on migrated devices
to include the proper app context. Freshly installed apps get this
done automagically, as do newly created files.

Change-Id: If7421dac01c6ae8bd78e66f71e69298ed850a7ab