Fix autotune compilation on OCaml < 4.13 (PR #772)

Author: sim642

src/autoTune.ml

@@ -83,7 +83,7 @@ let rec setCongruenceRecursive fd depth neigbourFunction =
          setCongruenceRecursive (Cilfacade.find_varinfo_fundec vinfo) (depth -1) neigbourFunction
       )
       (FunctionSet.filter (*for extern and builtin functions there is no function definition in CIL*)
-         (fun x -> not (isExtern x.vstorage || String.starts_with ~prefix:"__builtin" x.vname))
+         (fun x -> not (isExtern x.vstorage || BatString.starts_with x.vname "__builtin"))
          (neigbourFunction fd.svar)
       )
     ;