Bug#814606: libvigraimpex: FTBFS on armel (error: return type 'class std::future<void>')

[Ullrich Koethe]

> threadpool.hxx:333:26: error: return type 'class std::future<void>' is incomplete
>  ThreadPool::enqueue(F&& f)
>                           ^

This message is emitted by outdated compilers that lack sufficient C++11 
support. What compiler did you use? If gcc: was the '-std=c++11' flag 
active?

Best
Ulli (VIGRA maintainer)