games/keeperrl: unbreak with libc++ 7

In file included from effect.cpp:731:
./pretty_archive.h:135:12: error: call to 'quoted' is ambiguous

ar.os << std::quoted(t) << " ";
         ^~~~~~~~~~~

/usr/include/c++/v1/iomanip:645:1: note: candidate function [with _CharT = char, _Traits = std::1::char_traits<char>, _Allocator = std::1::allocator<char>]
quoted ( const basic_string <_CharT, _Traits, _Allocator> &s, _CharT delim = _CharT('"'), _CharT escape=_CharT('\\'))
^
./extern/iomanip.h:173:3: note: candidate function [with _CharT = char, _Traits = std::1::char_traits<char>, _Alloc = std::__1::allocator<char>]

quoted(const basic_string<_CharT, _Traits, _Alloc>& __string,
^

PR: 230355
Obtained from: upstream