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.