Unfortunately, this doesn't help, because pathconf is not present on mingw. So the best I can do is #define _POSIX_ to get the value of NAME_MAX that it has.
So given that the pathconf constants are available like this on mingw, perhaps there is something gnulib can do? I'm not sure what is best, whether simply to allow those constants to be found by defining _POSIX_, or to implement pathconf with them.
In any case: the information does already exist in some form, and there's no way to get at it portably with gnulib, that I can see.