I wonder if the source-to-source compilation could be used to transpile C++ code/libraries to other languages - e.g. could this be used (with appropriate modifications of course) to compile FLTK into Free Pascal code and used directly by Lazarus as a backend for LCL? Trying to use C++ libraries from non-C++ is always a PITA, especially if you want to avoid dynamic linking.

Also, since i mentioned Lazarus, if it can compile itself to Free Pascal, i wonder if it'd be useful for adding C++ support to Lazarus itself so that Free Pascal and C/C++ can be mixed in a project to make self-contained executables for desktop applications. It'd most likely need much more work than that just the compilation to make it a first class citizen like Free Pascal itself is for Lazarus/LCL (e.g. things like the object inspector and code tools being able to understand C++ well enough so that refactoring and stuff like doubleclicking on a button in a form automatically declaring and defining the handler and moving the editor cursor to the newly defined handler's code body), but maybe it could be used as a starting point.