Intel-C: declare support for gnu11

Without extensions, functions like `strdup` are not available since they
are actually controlled by feature flags such as _SVID_SOURCE and
_BSD_SOURCE. When using `-std=c11` on Intel, none of these flags are
set, so the functions are not declared properly leading to compile
errors.

Reported-by: Adam J. Stewart <[email protected]>
Closes: #16226

Modules/Compiler/Intel-C.cmake

@@ -16,7 +16,7 @@ endif()
 
 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}=c11")
+  set(CMAKE_C11_EXTENSION_COMPILE_OPTION "${_std}=gnu11")
 endif()
 
 if (NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 12.1)