Unless you are going to code it into Geany you need to make it a custom filetype, see http://www.geany.org/manual/current/index.html#custom-filetypes
Works like a charm. :-)
It would be cool to add a custom lexer, but I guess this would not be possible without C programming.
Is there a way to auto-build files? E.g. automatically run the build command after I save a file.