In the process of implementing Cell, I ended up writing this nifty set of path parsing functions along the way to replace the Allegro path API:
This turned into quite a robust set of routines, far beyond the limited set provided by Allegro. You have your basic concatenation, of course (path_rebase(), path_cat()), and I also added routines for normalization (path_collapse()), canonization against the file system (path_resolve(), which can also relativize), even a recursive path_mkdir(). When you're done building the path, just do path_cstr(path_obj) to get the final path string for use with file system functions.
Feel free to use it. :) Consider it MIT licensed like the rest of minisphere.