Add some c++11 flags in documentation

doc/Doxyfile.in

@@ -225,7 +225,10 @@ ALIASES                = "only_for_vectors=This is only for vectors (either row-
                          "note_try_to_help_rvo=This function returns the result by value. In order to make that efficient, it is implemented as just a return statement using a special constructor, hopefully allowing the compiler to perform a RVO (return value optimization)." \
                          "nonstableyet=\warning This is not considered to be part of the stable public API yet. Changes may happen in future releases. See \ref Experimental \"Experimental parts of Eigen\"" \
                          "implsparsesolverconcept=This class follows the \link TutorialSparseSolverConcept sparse solver concept \endlink." \
-                         "blank= "
+                         "blank= " \
+                         "cpp11=<span class='cpp11'>[c++11]</a>" \
+                         "cpp14=<span class='cpp14'>[c++14]</a>" \
+                         "cpp17=<span class='cpp17'>[c++17]</a>"
                          
 
 ALIASES += "eigenAutoToc=  "

doc/eigendoxy.css

@@ -166,6 +166,11 @@ div.toc ul {
   margin: 0.2em 0 0.4em 0.5em;
 }
 
+span.cpp11,span.cpp14,span.cpp17 {
+  color: #119911;
+  font-weight: bold;
+}
+
 /**** old Eigen's styles ****/