[DRE-maint] Bug#1066854: don't hard-code the name of the shared library

[Matthias Klose]

Package: src:ruby-magic
Version: 0.2.9-3
Severity: serious
Tags: sid trixie

the package build-depends on libmagic1, and depends on it. The name was 
recently changed to libmagic1t64.  Please don't hard-code it, but try to 
b-d on libmagic-dev, and derive the name of the shared library package 
from the libmagic-dev package.