This only applies to the long deprecated develop branch, other branches already contains all dependencies and don't require extra setup.
Hikari is based off LLVM 6.0 SVN, which API changes occasionally. Below is a list of SVN Revision IDs used:

• LLVM:321162
• Clang:321064
• LLD:321064
• Polly:321064
• compiler-rt:321064
• libcxxabi:321064
• libcxx:321064

libcxxabi and libcxx is required if you are using any form of C++ code
clang is the compiler frontend which is also required, apparently