[Ada] Issue errors on illegal contracts unless SPARK_Mode is Off

[Arnaud Charlet]

Illegal Global/Depends contracts should be flagged by frontend in code for
which SPARK_Mode is not specified, as GNATprove relies on contracts being
legal in those cases. The frontend should skip these errors only when
SPARK_Mode is Off. Now fixed, as shown on the following example.

Command:
--------
$ gcc -c notinspark.ads

Output:
-------
     1. package Notinspark is
     2.
     3.    function Get return Integer;
     4.
     5.    procedure Set with
     6.      Global => (In_Out => Get);
                                  |
        >>> global item must denote variable or state

     7.
     8. end Notinspark;

Tested on x86_64-pc-linux-gnu, committed on trunk

2014-10-10  Yannick Moy  <m...@adacore.com>

        * errout.adb (SPARK_Msg_N): Issue error unless SPARK_Mode is Off.


Index: errout.adb
===================================================================
--- errout.adb  (revision 216063)
+++ errout.adb  (working copy)
@@ -3138,7 +3138,7 @@
 
    procedure SPARK_Msg_N (Msg : String; N : Node_Or_Entity_Id) is
    begin
-      if SPARK_Mode = On then
+      if SPARK_Mode /= Off then
          Error_Msg_N (Msg, N);
       end if;
    end SPARK_Msg_N;