Added enum for cc65 exit codes. replaced stdlib exit code names constants in libsrc with cc65 exit code named constants

libsrc/geos-common/common/_afailed.c

@@ -28,5 +28,5 @@ void _afailed (char* file, unsigned line)
 
     DlgBoxOk(CBOLDON "ASSERTION FAILED", "PROGRAM TERMINATED" CPLAINTEXT);
 
-    exit (2);
+    exit (CC65_EXIT_AFAILED);
 }