[pdal] Remove MSVC 2015 Support?

[Howard Butler]

> On Jan 24, 2020, at 11:37 AM, Mateusz Loskot <mateusz at loskot.net> wrote:
> 
> Given the default C++ version in VS2017 is C++14, not C++17, technically,
> the switch does not mean PDAL would have to switch to C++17, just C++14.

Does /std:c++17 work in VS2017? Specifically in regard to filesystem, which is the only bcp'd bits we have left in the PDAL tree.

Howard