The problem arises from:
in qtdeclarative/tools/qmlcachegen/Qt5QuickCompilerConfig.cmake. This path is the installed binary dir, not the host tools. One probably needs a properly generated config file like e.g. the Qt5LinguistTools.cmake is, which writes the expected path name into the config file. The current content should then go into a Qt5QuickCompilerMacros.cmake to match the layout of other modules.