building/hadrian.md

@@ -26,6 +26,8 @@ $ ./boot && ./configure --with-intree-gmp
 $ hadrian/build -j
 ```
 
+If the above fails on macOS with `fatal error: 'ffi.h' file not found` run `export C_INCLUDE_PATH="$(xcrun --show-sdk-path)/usr/include/ffi"` and then configure again.
+
 On Windows:
 
 ``` sh