server-mode: Do not crash when running into INTERFACE_LIBRARY
Please consider this for the release branch!
This is a bit ugly, but I did not want to get into major surgery to properly support
INTERFACE_LIBRARY in the release branch.
Ideally there would be a way to report the include directories/defines that are exported by this kind of target, so that I can scan for the include files that are part of this.
On the other hand: Maybe this is not really necessary since the targets linking against interface libraries do report the include paths that they got from the interface library.
Closes: #16539 (closed)