IntelLLVM: C17 support

Issue: #17755

Modules/Compiler/IntelLLVM-C.cmake

@@ -37,6 +37,9 @@ if(NOT "x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
 
   set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c11")
   set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
+
+  set(CMAKE_C17_STANDARD_COMPILE_OPTION "-std=c17")
+  set(CMAKE_C17_EXTENSION_COMPILE_OPTION "-std=gnu17")
 else()
   # clang-cl doesn't have any of these
   set(CMAKE_C90_STANDARD_COMPILE_OPTION "")
@@ -47,12 +50,13 @@ else()
 
   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(NOT "x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
-  # FIXME: The compiler actually defaults to C17, but
-  # CMake does not yet model or detect that standard.
-  __compiler_check_default_language_standard(C 2020 11)
+  __compiler_check_default_language_standard(C 2020 17)
 else()
   set(CMAKE_C_STANDARD_DEFAULT "")
 endif()