www/wt: unbreak with boost 1.63

In file included from wt-3.3.6/src/Wt/Render/CssParser.C:26:
[...]
/usr/local/include/boost/phoenix/function/detail/cpp03/preprocessed/function_operator_10.hpp:91:105: error: cannot initialize a parameter of type 'char *' with an lvalue of type 'char const[17]'

return detail::expression::function_eval<F, A0 , A1 , A2 , A3 , A4>::make(f, a0 , a1 , a2 , a3 , a4);
                                                                                            ^~

PR: 215599
Approved by: Mohammad S. Babaei (maintainer)