Intel: Correct the C11 ext flag

Intel does not support a 'gnu11' standard flag, only 'c11'.

Modules/Compiler/Intel-C.cmake

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