Fixed compilation warnings.  Added -Wall to linux-gcc compilation.  JSON_ASSERT_MESSAGE now throws exception (but JSON_ASSERT does not).
diff --git a/include/json/value.h b/include/json/value.h
index de09d5f..480f433 100644
--- a/include/json/value.h
+++ b/include/json/value.h
@@ -518,7 +518,7 @@
    class ValueAllocator

    {

    public:

-      enum { unknown = -1 };

+      enum { unknown = (unsigned)-1 };

 

       virtual ~ValueAllocator();