How can I portably call a C++ function that takes a char** on some platforms and a const char** on others?

If what you want is just to turn a blind eye to some const issues, then you can use a conversion which blurs the distinction, i.e. makes char** and const char** interoperable:

template<class T>
class sloppy {}; 

// convert between T** and const T** 
template<class T>
class sloppy<T**>
{
    T** t;
    public: 
    sloppy(T** mt) : t(mt) {}
    sloppy(const T** mt) : t(const_cast<T**>(mt)) {}

    operator T** () const { return t; }
    operator const T** () const { return const_cast<const T**>(t); }
};

Then later in the program:

iconv(c, sloppy<char**>(&in) ,&inlen, &out,&outlen);

sloppy() takes a char** or a const char* and converts it to a char** or a const char*, whatever the second parameter of iconv demands.

UPDATE: changed to use const_cast and call sloppy not a as cast.