Add a check that a simple source file can compile as C++17 that uses
some of the features we need. Do this only when hosted by CMake 3.8
or above because those versions are aware of C++17.
Check for unordered_map as we do in bootstrap since commit 375eca7881
(bootstrap: Check support for unordered_map from compiler mode,
2017-11-30). Also maintain the existing C++14 cstdio check.
