Are these libraries at all portable? Do I have to be on Linux or KDE to make use of these? The feature set of eg Solid sounds very tricky to implement in a cross-platform way.
Making it possible to use the functionality outside of KDE applications is the entire point of splitting it into a bunch of standalone libraries. http://api.kde.org/frameworks-api/frameworks5-apidocs/ says Solid is supported on Linux, Windows and OS X, so I assume they've just implemented all of the platform-specific versions needed.