No known features for CXX compiler "Clang" version 9.0.1.
quassel 1.13.1 does not compile on macOS with open-source clang 9.0.1, as installed by MacPorts:
CMake Error in src/CMakeLists.txt: No known features for CXX compiler "Clang" version 9.0.1.
It is necessary to use a newer open-source clang to compile quassel on older versions of OS X because your code requires thread-local storage which older Apple clangs did not provide.
But it is easy to verify the problem even on newer versions of macOS by attempting to compile using open-source clang 9.0.1 from MacPorts instead of Xcode clang from Apple.
The Version popup menu in this issue tracker is missing version 0.13.1; please add it.