Additional information for those who needs to deal with custom(ized) build time dependencies as separate packages.
Custom build time dependencies can be conveniently installed with the help of the search-output-dir configuration option of sfdk (Qt Creator uses sfdk for building). This option is best combined with output-prefix <dir> option, which also implies it:
mkdir -p ~/tmp/RPMS
sfdk config --global --push output-prefix "$HOME/tmp/RPMS"
With this configuration, sfdk will store all RPMs in subdirectories of ~/tmp/RPMS and it will also search those directories for build time dependencies. See sfdk --help-building for more information.
Note that Qt Creator builds the RPMs only during the deployment step, not during the build step. This means that the library RPMs will not be created until the library is deployed. You may configure your library project to use the “Build RPM Packages for Manual Deployment” deployment method to avoid the need to actually deploy the library when you only need it as a build time dependency.