Use cil.cstd option for preprocessing as well

Author: sim642

src/config/options.schema.json

@@ -279,7 +279,7 @@
         "cstd": {
           "title": "cil.cstd",
           "type": "string",
-          "description": "Specify the c standard used for parsing.",
+          "description": "Specify the c standard used for preprocessing and parsing.",
           "default": "c99",
           "enum": [
             "c89", "c90",

src/maingoblint.ml

@@ -267,6 +267,8 @@ let preprocess_files () =
   (* Preprocessor flags *)
   let cppflags = ref (get_string_list "pre.cppflags") in
 
+  cppflags := ("--std=" ^ get_string "cil.cstd") :: !cppflags;
+
   if get_bool "ana.sv-comp.enabled" then (
     let architecture_flag = match get_string "exp.architecture" with
       | "32bit" -> "-m32"