Comment out -Wextra because it doesn't work on older versions of gcc.

[SVN r39009]

test/Jamfile.v2

@@ -8,7 +8,8 @@ import testing ;
 project hash-tests
     : requirements
         <toolset>gcc:<define>_GLIBCXX_DEBUG
-        "<toolset>gcc:<cxxflags>-Wsign-promo -Wextra"
+        <toolset>gcc:<cxxflags>-Wsign-promo
+        #<toolset>gcc:<cxxflags>-Wextra
     ;
 
 test-suite functional/hash