Bug#1066851: don't hard-code the name of the shared library

[Matthias Klose]

Package: src:raku-readline
Version: 0.1.6-3
Severity: serious
Tags: sid trixie

the package build-depends on libreadline8, and depends on it. The name 
was recently changed to libreadline8t64.  Please don't hard-code it, but 
try to b-d on libreadline-dev, and derive the name of the shared library 
package from the libreadline-dev package.