Clang: C17 support

Added in LLVM commit 5b6c0f75e01571851b767dc63a3229c962f464f1, available since
Clang 6.

Issue: #17755

Modules/Compiler/Clang-C.cmake

@@ -41,6 +41,11 @@ if(NOT "x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
     set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c1x")
     set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu1x")
   endif()
+
+  if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 6.0)
+    set(CMAKE_C17_STANDARD_COMPILE_OPTION "-std=c17")
+    set(CMAKE_C17_EXTENSION_COMPILE_OPTION "-std=gnu17")
+  endif()
 else()
   set(CMAKE_C90_STANDARD_COMPILE_OPTION "")
   set(CMAKE_C90_EXTENSION_COMPILE_OPTION "")
@@ -48,6 +53,8 @@ else()
   set(CMAKE_C99_EXTENSION_COMPILE_OPTION "")
   set(CMAKE_C11_STANDARD_COMPILE_OPTION "")
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_C17_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_C17_EXTENSION_COMPILE_OPTION "")
 endif()
 
 if(CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 2.1)