feat: conditionally enable mmap functionality

[finger563]

• Only enable include of csv2/mio.hpp in csv2/reader.hpp if sys/mman.h or windows.h can be found
  This helps on platforms such as ESP32 which do not have mmap support exposed through sys/mman.h (though they do have memory mapping support at a lower layer, so perhaps that could be utilized via a later PR).

Tested on ESP32 using the following code:

    //! [csv reader example]
    std::string csv_data =
      "filename, boxart filename, display name\n"
      "mario.nes, boxart/mario.jpg, Mario Bros.\n"
      "super_mario_1.nes, boxart/super_mario_bros_1.jpg, Super Mario Bros.\n"
      "super_mario_3.nes, boxart/super_mario_bros_3.jpg, Super Mario Bros. 3\n"
      "zelda.nes, boxart/zelda1.jpg, The Legend of Zelda\n"
      "zelda_2.nes, boxart/zelda2.jpg, The Legend of Zelda 2: the Adventure of Link\n"
      "mega_man.nes, boxart/megaman1.jpg, MegaMan\n"
      "metroid.nes, boxart/metroid1.jpg, Metroid\n"
      "pokemon_blue.gb, boxart/pokemon_blue.jpg, Pokemon Blue\n"
      "pokemon_red.gb, boxart/pokemon_red.jpg, Pokemon Red\n"
      "pokemon_yellow.gbc, boxart/pokemon_yellow.jpg, Pokemon Yellow\n"
      "links_awakening.gb, boxart/tloz_links_awakening.jpg, The Legend of Zelda: Link's Awakening\n"
      "links_awakening.gbc, boxart/tloz_links_awakening_dx.jpg, The Legend of Zelda: Link's Awakening DX";

    // create the csv reader object
    csv2::Reader<csv2::delimiter<','>,
                 csv2::quote_character<'"'>,
                 csv2::first_row_is_header<true>,
                 csv2::trim_policy::trim_whitespace> csv;
    if (csv.parse(csv_data)) {
      // print the header
      const auto header = csv.header();
      fmt::print("Header:\n\t");
      for (const auto cell: header) {
        std::string value;
        cell.read_value(value);
        fmt::print("'{}', ", value);
      }
      fmt::print("\n");
      // now print the items
      size_t row_index = 0;
      for (const auto row: csv) {
        fmt::print("Row {} values:\n\t", row_index);
        for (const auto cell: row) {
          std::string value;
          cell.read_value(value);
          fmt::print("'{}', ", value);
        }
        fmt::print("\n");
        row_index++;
      }
    }
    //! [csv reader example]

With the following output: