Creating a Swift framework around a C library

Using Xcode, I'm trying to build a Swift framework around libmodbus, a C library for communicating with MODBUS devices.

But Xcode doesn't let me include modbus.h in the bridging header, saying

<unknown>:0: error: using bridging headers with framework targets is unsupported

How should I go about doing this? This seems like a pretty severe (and arbitrary) limitation.