Mark SunPro 5.3 unusable, because it is

[SVN r27582]

status/explicit-failures-markup.xml

@@ -438,6 +438,7 @@
             <toolset name="borland-5_6_4"/>
             <toolset name="msvc"/>
             <toolset name="msvc-stlport"/>
+            <toolset name="sunpro-5_3"/>
         </mark-unusable>
         <mark-expected-failures>
             <test name="adj_matrix_cc"/>