Document that libetpan now uses pkg-config

[pmetzger]

Howdy! Although the last update only bumped from 1.9.3 to 1.9.4, the package now uses an incompatible configuration system. This wasn't noted in any release notes, and has broken builds with dependent packages (like for example, claws-mail, which used the old system during autoconfiguration.) It would be useful to call such changes out in release notes...