_M_underflow_common missing

[Benjamin Kosnik]

Yo.

std::basic_filebuf<unsigned char, std::char_traits<unsigned
char>>::_M_underflow_common(bool)

Notice that this is not a required instantiation
(basic_filebuf<unsigned char>). It's required to compile, but not
necessarily link.

There is a list of member functions, locale facets, etc that must be
provided for fully-formatted IO to work with non-required types. I'll
add documenting this to the TODO list.

In the meantime, I suggest you hack up a version of
_M_underflow_common (found in src/fstream.cc) for unsigned char and
link it in...

-benjamin