[boost] [outcome] Fix with support for VS2017 Update 1

Earlier today Vinnie Falco found that the peer review edition of Outcome does not compile on Visual Studio 2017 Update 1. I do apologise for this. Update 1 came out a few days before the peer review edition was set in stone. I didn't know it had until now, and I tested against VS2017 RTM, not Update 1. I have applied a fix to develop branch and cherry picked just this fix to the frozen master branch. You can find that fixed peer review edition at this branch: https://github.com/ned14/boost.outcome/tree/master_vs2017_fixed I hope that this does not inconvenience people too much, and once again, sorry. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/