My definition of portable is that it can create code that will compile across all C++ compilers without hacks and ifdefs.
I really care little about GNU and open source. Sure, it's great, but it's not a requirement for a software to be free. As long as the software works, does its purpose, and I can get help when I need it, it's enough for me.Depends on your definition of "free". http://www.gnu.org/philosophy/free-sw.html
(I would likely have said "free" as well, BTW.)
If it costs nothing, then it's free.