[CI] fix build jobs with INCLUDE_SPLIT
- Related issues: #885 (closed)
- Requested reviewers: @pajunen @pihlajakuja
The split rendering build jobs are supposed to test the build with split rendering activated, but they actually only test INCLUDE_SPLIT being passed to make/CMake. The define SPLIT_REND_WITH_HEAD_ROT
needs to be activated additionally for the test to be meaningful. This should also explain the warnings about empty object files in #885 (closed).
Doing the correct build here results in a range of warnings which should be addressed as well.