A bug report from Ludovico Massaccesi on the dev email list indicates that the build is failing because of a deprecation warning about ClassImp from ROOT, which is being promoted to an error because of -Werror. We, strangely, don't see such warnings in our regular CI builds but nevertheless this should clearly be fixed.
The ROOT manual pages have been recently updated on the topic of dictionary generation, see:
Things are now much more clearly explained, so we should adapt accordingly.
Firstly, ClassImp is indeed no longer needed, so we should just remove it. This should be done in v3r5-patches and then we release v3r5p2. The change should also be made in master and propagated to timedep-branch.
The ROOT_GENERATE_DICTIONARY CMake macro has been improved so that the old-style include_directories should not be needed anymore - the necessary changes should be implemented in master and propagated to timedep-branch. This should be tested against ROOT older versions and, if necessary, we can bump the minimum version (currently 6.14 in master).
One important clarification in the recent ROOT manual changes is that dictionaries are only needed for classes that require I/O, so we can actually stop generating dictionaries for most of our classes - we should try this out in master.
Once we're happy with the state of master we can make a v3r6.