Mark up math lib tests that require external software in order to pass.

[SVN r55109]

status/explicit-failures-markup.xml

@@ -3260,6 +3260,14 @@ for more information.
               many issues to stand a chance of porting to this compiler.
            </note>
         </mark-unusable>
+       <mark-expected-failures>
+          <test name="mpfr_concept_check"/>
+          <test name="ntl_concept_check"/>
+          <toolset name="*"/>
+          <note author="John Maddock">
+             This test relies on external software being installed in order to pass.
+          </note>
+       </mark-expected-failures>
        <mark-expected-failures>
           <test name="test_traits"/>
           <toolset name="gcc-3.3.6"/>