[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 (closed).