Fail to build with boost 1.61

System is Debian unstable i386

```
src/accumulator.hpp: In instantiation of 'accumulator_type* accumulator(T*) [with T = int]':
src/main.cpp:569:46: required from here
src/accumulator.hpp:97:12: error: invalid new-expression of abstract class type 'accumulator_type'
return new accumulator_type(store);
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/accumulator.hpp:18:7: note: because the following virtual functions are pure within 'accumulator_type':
class accumulator_type : public boost::program_options::value_semantic
^~~~~~~~~~~~~~~~
In file included from /usr/include/boost/program_options/options_description.hpp:13:0,
from /usr/include/boost/program_options.hpp:15,
from src/main.cpp:26:
/usr/include/boost/program_options/value_semantic.hpp:44:22: note: virtual bool boost::program_options::value_semantic::adjacent_tokens_only() const
virtual bool adjacent_tokens_only() const = 0;
^~~~~~~~~~~~~~~~~~~~
Makefile:452: recipe for target 'src/main.o' failed
```

Start with the compiler error in src/accumulator.hpp and the Boost value_semantic declaration referenced in the report. Reproduce the build on Debian unstable i386 with Boost 1.61, then verify that the project builds successfully and src/main.cpp compiles.

Written by the indexing model from the issue text.