If you have libclang installed and Leon can't find it, please open an issue.
Otherwise, if you still need to provide your own libclang, put your libclang bin lib and include in here.
bin should contain libclang.dll, libclang.so, or something similar, depending on your target system.
lib should contain libclang.lib, libclang.a, or something similar, depending on your target system.
include should contain clang-c.