Looks like the pro2cmake.py translation made a bit of a mess of the file at qtbase/examples/widgets/tools/plugandpaint/app/CMakeLists.txt. There's invalid CMake syntax in various places and it looks like the translation went a bit haywire:

if(macx-xcode)
    target_link_libraries(plugandpaint PUBLIC
        (
        )
        pnp_basictools$
    )
endif()

if(NOT macx-xcode)
    target_link_libraries(plugandpaint PUBLIC
        pnp_basictools
    )
endif()

if(((NOT (macx-xcode)) AND (( NOT debug_and_release OR build_pass ) AND CONFIG(debug,debug OR release))) AND (APPLE))
    target_link_libraries(plugandpaint PUBLIC
        (LIBS, 0)
        (LIBS, 1)
        _debug
    )
endif()

if(((NOT (macx-xcode)) AND (( NOT debug_and_release OR build_pass ) AND CONFIG(debug,debug OR release))) AND (WIN32))
    target_link_libraries(plugandpaint PUBLIC
        (LIBS, 0)
        (LIBS, 1)
        d
    )
endif()

A secondary issue is that this CMakeLists.txt file refers to the pnp_basictools library, but that is built by a sibling directory. When the app example is built on its own, it can't find the pnp_basictools library because it isn't part of the same build.

I'm wondering if the app and plugins are supposed to be separate example projects or whether they should be a single project. There seems to be other examples that do it both ways, so there isn't a single consistent convention for this.