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.

1 files changed, 7 insertions, 0 deletions

diff --git a/Source/Checks/cm_cxx17_check.cpp b/Source/Checks/cm_cxx17_check.cpp
new file mode 100644
index 0000000..2cbf1d5
--- /dev/null
+++ b/Source/Checks/cm_cxx17_check.cpp
@@ -0,0 +1,7 @@
+#include <cstdio>
+#include <unordered_map>
+
+int main()
+{
+  return 0;
+}