Brad King 31d59ff3b1 Compile CMake as C++17 if supported by the compiler
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.
2017-12-22 09:05:54 -05:00
..