It looks at the same folder where that particular .rc file is located. This is not in the control of X#, it's the resource compiler (rc.exe) that handles .rc compilation, I checked if it supports additionally looking at some general pool folder for external files, but unfortunately it seems it doesn't.
If you are compiling inside VS then the last commandline for the call to the native resource compiler is stored in your <LocalAppdata>Temp folder. I think it is called LastXSharpNativeResourceResponseFile.Rsp In the same way the last response file for the compiler is stored in that folder as LastXSharpResponseFile.rsp.
You will see that the include path that is specified under the preprocessor settings is also included in the call to the native resource compiler as well as the defines defined there.
IIRC the native resource compiler is called with a current directory equal to the project directory.
This is controlled by the XSharp.Build DLL for which you can find the source at: github.com/X-Sharp/XSharpPublic/tree/mas...lStudio/XSharp.Build
XSharp Development Team