Intel: Fix incorrectly documented extension flags

Fixes: #18166

Modules/Compiler/Intel-C.cmake

@@ -25,8 +25,7 @@ else()
 
   if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 15.0.0)
     set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c11")
-    # todo: there is no gnu11 value supported; figure out what to do
-    set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=c11")
+    set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
   endif()
 
   if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 12.0)

Modules/Compiler/Intel-CXX.cmake

@@ -36,18 +36,21 @@ else()
 
   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 18.0.0)
     set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std=c++17")
-    # todo: there is no gnu++17 value supported; figure out what to do
-    set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=c++17")
+    set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++17")
   endif()
 
   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 15.0.2)
     set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++14")
-    # todo: there is no gnu++14 value supported; figure out what to do
-    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=c++14")
   elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 15.0.0)
     set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++1y")
-    # todo: there is no gnu++14 value supported; figure out what to do
-    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=c++1y")
+  endif()
+
+  # Intel 15.0.2 accepts c++14 instead of c++1y, but not gnu++14
+  # instead of gnu++1y.  Intel 17.0.0 accepts gnu++14 too.
+  if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 17.0)
+    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++14")
+  elseif (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 15.0.0)
+    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
   endif()
 
   if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 13.0)