configure: bump minimal OCaml version to 4.07

So that we can use `Stdlib.xxx` function names.

Author: xavierleroy

configure

@@ -581,14 +581,14 @@ fi
 echo "Testing OCaml... " | tr -d '\n'
 ocaml_ver=$(ocamlc -version 2>/dev/null | tr -d '\r')
 case "$ocaml_ver" in
-  4.0[5-9].*|4.1?.*|5.*)
+  4.0[7-9].*|4.1?.*|5.*)
         echo "version $ocaml_ver -- good!";;
   ?.*)
         echo "version $ocaml_ver -- UNSUPPORTED"
         if $ignore_ocaml_version; then
             echo "Warning: this version of OCaml is unsupported, proceed at your own risks."
         else
-            echo "Error: make sure OCaml version 4.05 or more recent is installed."
+            echo "Error: make sure OCaml version 4.07 or more recent is installed."
             missingtools=true
         fi;;
   *)