Build with "--input custom" re2c switch.

These are three examples of "--input custom" usage:

- input_custom_default.re:
    implements default re2c input model (pointers to plain buffer)

- input_custom_fgetc:
    implements C-style file input (using <stdio.h>)

- input_custom_fgetc:
    implements std::istringstream input

Note that these examples are very simple and don't need
to implement YYFILL; the only reason they don't use
"re2c:yyfill:enable = 0;" is to keep YYLESSTHAN and YYLIMIT
(for the sake of example).

In real-life programs one will need to care for correct
end-of-input handling.