## C++17 is now required for VTK

VTK now permits C++17 language features and hence requires C++17 compiler support. The minimum required compiler versions have been updated as well.

* Updates minimum required compilers:
    * gcc 8.0 from 4.8
    * LLVM Clang 5.0 from 3.3
    * Apple Clang 10.0 from 7.0
    * MSVC 2017 from 2015
    * Intel ICC 19.0 from 14.0
