prefer libs build from source over the system

I'm working on an Alien::Base dist, and it works with the system package and it works building from source, but if I install from source and then install the system packages, it stops working with this error:

```
t/list_contents_of_archive.t .. Can't load '/home/ollisg/dev/Archive-Libarchive-XS/.build/2FkGCFgZ1T/blib/arch/auto/Archive/Libarchive/XS/XS.so' for module Archive::Libarchive::XS: libarchive.so.13: cannot open shared object file: No such file or directory at /home/ollisg/perl5/perlbrew/perls/perl-5.18.1/lib/5.18.1/x86_64-linux/DynaLoader.pm line 190.
 at t/list_contents_of_archive.t line 3.
```

This patch prefers the Alien::Base lib dir to the system one, and makes it work for me.