have an explicit define for OS4, so we can easily identify it in shared sources among Amiga-like systems, independent from the CPU define

git-svn-id: trunk@26859 -

compiler/systems/i_amiga.pas

@@ -99,7 +99,7 @@ unit i_amiga;
             flags        : [tf_files_case_aware];
             cpu          : cpu_powerpc;
             unit_env     : '';
-            extradefines : 'PPC603;HASAMIGA';
+            extradefines : 'PPC603;HASAMIGA;AMIGAOS4';
             exeext       : '';
             defext       : '.def';
             scriptext    : '.sh';