What is the proper way to make library pin mappings configurable?
The method I use is to provide the pins as parameters to the constructor. Those pin numbers are stored in variables to use later in the .begin() function and elsewhere.

Most of the time I use initialization lists to keep things simple. For example:

class Something {
    uint8_t _cs;
    uint8_t _dc;

    Something(uint8_t cs, uint8_t dc) : _cs(cs), _dc(dc) {}
    void begin();
};

void Something::begin() {
    pinMode(_cs, OUTPUT);
    pinMode(_dc, OUTPUT);
}

Something mySomething(10, 8);