Intel-C: support gnu89 and gnu99 extension flags

Modules/Compiler/Intel-C.cmake

@@ -21,9 +21,9 @@ endif()
 
 if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 12.1)
   set(CMAKE_C90_STANDARD_COMPILE_OPTION "${_std}=c89")
-  set(CMAKE_C90_EXTENSION_COMPILE_OPTION "${_std}=c89")
+  set(CMAKE_C90_EXTENSION_COMPILE_OPTION "${_std}=gnu89")
   set(CMAKE_C99_STANDARD_COMPILE_OPTION "${_std}=c99")
-  set(CMAKE_C99_EXTENSION_COMPILE_OPTION "${_std}=c99")
+  set(CMAKE_C99_EXTENSION_COMPILE_OPTION "${_std}=gnu99")
 endif()
 
 if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 12.1)