Specifying C include paths to swift build?

I've run into this issue before. My app depends on a Swift library that wraps libmodbus, which is a C library. I had to customize that, so I built it from sources, and it's installed in /usr/local/lib and /usr/local/include/modbus. swift build can't find modbus.h. How do I tell Swift to look in /usr/local/lib/modbus when building?

UPDATE: Nevermind, it's fine, but I can't delete this question (afaict)