OK panic over I finally found the incredibly obvious and incredibly dumb solution in this article http://edn.embarcadero.com/article/22694
] which in summary is, add a file to the .exe project as if you were adding an existing source file, only fiddle the file extension so it shows .lib files and add the built static lib. Sadly this means there is no way to actually control the linker command line directly, no equivalent of --nodefault libs and Code Gear/RAD Studio/CppBuilder is technology locked to their own libraries. Even Microsoft and Apple don't do that. This means I'll have to drop support for it from my project :-( but at least I think I've got the bottom of the issue.
I wait in hope to be corrected by any CodeGear RAD Studio users out there who know a better way.