Mark borland 5.6.* and 5.8.* as unusable for asio.

[SVN r40382]

status/explicit-failures-markup.xml

@@ -147,6 +147,18 @@
         </test>
     </library>
 
+    <!-- asio -->
+    <library name="asio">
+      <mark-unusable>
+        <toolset name="borland-5.6*"/>
+        <toolset name="borland-5.8*"/>
+        <note author="Chris Kohlhoff">
+            This compiler does not support enable_if, which is needed by the
+            Boost.System library on which Boost.Asio depends.
+        </note>
+      </mark-unusable>
+    </library>
+
     <!-- assign -->
     <library name="assign">
         <mark-unusable>