Debian builds: use compiled librtlsdr
This release concerns only Debian builds. The dependency to distribution packaged librtlsdr has been removed in favor of a locally packaged librtlsdr compiled from source (part so called "external libraries") on a definite version that fits with the rest of the code. This should solve problem with Debian Buster build as reported in #455 and will serve in the future if a special version of RTL-SDR library is needed.