The Qt profile generated by 'qbs-setup-qt' sets the 'libPath' property of the 'Qt/core/core.qbs' mopdule to "/usr/lib64", which than later in the module gets written into 'cpp.libraryPaths'.
This makes the compiler search for every library first inside "/usr/lib64", regardless of any other paths I have added to 'cpp.libraryPaths'. Any product using Qt and a library that has an incompatible version installed on the system can not be linked.
The 'libPath' property of the core module does not seem to be needed when Qt is installed in a default library path. Setting 'libPath' to "/dev/null" fixes the problem.