The various logging levels are available at the start of C files. It is close to the Qemu approach, replacing the LOG function by the va_args version of logging().
You can then enable the logs of a file by defining the DEBUG_ variable at the start of the file. The #define is usually left commented, thus you only need to uncomment it.
The current list of logs is available below:
% grep -Rn "define DEBUG" . | egrep -v "(z3|capstone|qemu)"
./disassemblers/disassembler_xed.c:9://#define DEBUG_XED
./disassemblers/disassembler_meta.c:11://#define DEBUG_META
./converter.c:7://#define DEBUG_LLVM
./chain.c:11://#define DEBUG_CHAIN
./map.c:7:#define DEBUG_Z3_SOLVE
./plugins/plugin_rop.c:7://#define DEBUG_ROP
./plugins/plugin_rop.c:14://#define DEBUG_ROP_DEBUG