clangdb generator fails because of input files outside the project

Day and night I dream about using the clang tools and all other kinds of tools with the Qt Creator source code. One hot feature of qbs is, therefore, the clangb generator. Unfortunately, it fails on the Qt Creator source tree because of a cpp file which should not even be in the build graph:

c:\Users\aportale\dev\qt-creator>qbs generate -g clangdb profile:qt-5-9-1

Restoring build graph from disk
Resolving project for configuration default
Building for configuration default
Build done for configuration default.
ERROR: No rule was found that produces an artifact tagged 'obj' from input file 'C:/Users/aportale/dev/qt-creator/doc/api/examples/exampleplugin/exampleplugin.cpp'.

With the msvc generator, it works:

c:\Users\aportale\dev\qt-creator>qbs generate -g visualstudio2015 profile:qt-5-9-1

Restoring build graph from disk
Resolving project for configuration default
Building for configuration default
Build done for configuration default.
Generated Qt Creator.sln