Wikipedia:
Suppose my program relies on glib to implement file copies. I want to compile my code to run on FooBox, a platform glib does not support. It seems I'll need to write some new code, rather than use the code I already have. Ergo, non-portable.Portability is the software codebase feature to be able to reuse the existing code instead of creating new code when moving software from an environment to another.
I'm not saying that glib (or something else) isn't a good solution, just that introducing a third party library doesn't magically make code portable.