Fixed installation when changing DESTDIR

Fixed installation when changing DESTDIR

After changing the DESTDIR the installated binaries have some issues
due to hard coded paths. For example, using distributions on NetEm
would segfault.

I've changed iplink.c and tc_util.c so they are now aware of DESTDIR.
Along with that change I needed to change the main Makefile so it
defines the DESTDIR macro when calling gcc.

I also changed the paths so that during the installation sbin, etc,
share and lib directories are created directly inside of the DESTDIR,
instead of creating a usr directory inside that. That's the behaviour
of most packages out there, so I think most users will be expecting
that to happen.