Based on comments here (https://github.com/kogmbh/WebODF/pull/406#discussion_r9900297), CMake documentation seems to suggest manifest.json-target isn't actually valid to use here, as custom targets can only depend on other file outputs NOT other targets. Seems to work regardless, but @kossebau has been using syntax more like:
```
add_custom_target(qtjsruntimetest ALL ...)
add_dependencies(qtjsruntimetest manifest.json-target)
```
I'm torn as to whether to follow the CMake doc suggestions, or fall back on the "this appears to currently work" excuse.