Point is... it's a gray area.
Linking technology itself is actually rather irrelevant to the legal questions that a court general ask in defining copyright. FSF happen use it as a bright line in the sand on where they will enforce the license, but any author can make their own decision on that matter. Game companies has for example gone after mods that inject itself through windows libraries, arguing that the mods purpose and existence is dependent on the original game and thus created a derivate work.
There was a historical case when a person legally bought a painting and cut it into several pieces only to stitched them together into a mosaic. The court judged it as a derivate work and thus requiring additional copyright permission from the author.
As with other gray areas in copyright, derivative work also have semi-exceptions. Compatibility is one. Since there is multiple different C standard libraries, it would be hard to argue that a program is specifically a derivative work of one. It would also be very easy to demonstrate the works independence by simply using a different standard library.
PyQt brings a interesting question but in the end I doubt it would be that much of a gray area. In theory you could make a program that is only compatible with Qt but don't actually depend on it, but in practice I suspect most such program do not start, operate, and function if you remove the Qt parts. In such cases I expect the accused will have a hard time arguing that the final work when used has no copyright-protected elements of Qt.