Make sure `std::cbegin`, `std::cend`, and `std::size` work in C++17 or
C++14 mode before choosing the corresponding standard level for
compiling CMake itself. This helps in cases that the compiler is using
a standard library too old to support the full standard level chosen.
GUID functions in ATL cause compilation errors with this compiler.
Add the offending case to our check for C++17 support so that it
is not used by default when it does not work.
When testing compiler modes higher than C++11 for constructs we need,
include a check for using `unique_ptr` in that mode. The PGI 18.4
compiler in some environments supports `unique_ptr` in C++11 mode
but is broken for C++14 and C++17. Check that `unique_ptr` works
in these modes before using one.
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.
