Don't define abs_* variables ourselves

Don't define abs_* variables ourselves

Apparently this was necessary in the past because old versions
of autoconf/automake didn't make them available, but these
days all of the platforms we target include recent enough
autotools - as evidenced by the fact that, for example, we
already use abs_top_srcdir in tools/ despite the fact that
tools/Makefile.am is missing the same boilerplate.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>