Header file not found in Eclipse CDT

I'm trying to build a project that uses an external library (Casablanca) using Eclipse CDT 8.8.1. on Mac OS. As expected, the first build run returned "cpprest/http_client.h file not found" errors.

I then proceeded to add an include path (/Users/me/projects/casablanca/Release/include) by right-clicking on the project, selecting Properties->C/C++ Include Paths and Symbols, Add External Include Path, and moving it up in priority as the first listed folder.

However this did not solve the problem and Eclipse still can't seem to find the headers. The #include directive is as below:

#include <cpprest/http_client.h>

and the added /include directory does have a /cpprest sub-directory, with http_client.h in it.

Any help would be appreciated.