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.

2 files changed, 43 insertions, 0 deletions

diff --git a/Source/Checks/cm_cxx17_check.cmake b/Source/Checks/cm_cxx17_check.cmake
new file mode 100644
index 0000000..83d3971
--- /dev/null
+++ b/Source/Checks/cm_cxx17_check.cmake
@@ -0,0 +1,36 @@
+set(CMake_CXX17_BROKEN 0)
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
+  if(NOT CMAKE_CXX17_STANDARD_COMPILE_OPTION)
+    set(CMake_CXX17_WORKS 0)
+  endif()
+  if(NOT DEFINED CMake_CXX17_WORKS)
+    message(STATUS "Checking if compiler supports needed C++17 constructs")
+    try_compile(CMake_CXX17_WORKS
+      ${CMAKE_CURRENT_BINARY_DIR}
+      ${CMAKE_CURRENT_LIST_DIR}/cm_cxx17_check.cpp
+      CMAKE_FLAGS -DCMAKE_CXX_STANDARD=17
+      OUTPUT_VARIABLE OUTPUT
+      )
+    if(CMake_CXX17_WORKS AND "${OUTPUT}" MATCHES "error: no member named.*gets.*in the global namespace")
+      set_property(CACHE CMake_CXX17_WORKS PROPERTY VALUE 0)
+    endif()
+    if(CMake_CXX17_WORKS)
+      message(STATUS "Checking if compiler supports needed C++17 constructs - yes")
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeOutput.log
+        "Determining if compiler supports needed C++17 constructs passed with the following output:\n"
+        "${OUTPUT}\n"
+        "\n"
+        )
+    else()
+      message(STATUS "Checking if compiler supports needed C++17 constructs - no")
+      file(APPEND ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/CMakeError.log
+        "Determining if compiler supports needed C++17 constructs failed with the following output:\n"
+        "${OUTPUT}\n"
+        "\n"
+        )
+    endif()
+  endif()
+  if(NOT CMake_CXX17_WORKS)
+    set(CMake_CXX17_BROKEN 1)
+  endif()
+endif()
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;
+}