[Vc] Work around missing std::for_each_n() in g++ 7.2.

`Vc/common/algorithm.hh` uses `std::for_each_n()` when it detects C++17, but
g++ 7.2 does not seem to provide it (yet).  Work around this by disabling the
macro that signals C++17-support in Vc.

Closes: #88.