Remove reference to deprecated std::binary_function (#1068)

C++11 deprecates std::binary_function. C++17 removes it.
It only provides typedefs that are unused since C++11.

src/HttpHeaderTools.h

@@ -67,7 +67,7 @@ class HeaderManglers
 private:
     /// Case-insensitive std::string "less than" comparison functor.
     /// Fast version recommended by Meyers' "Effective STL" for ASCII c-strings.
-    class NoCaseLessThan: public std::binary_function<std::string, std::string, bool>
+    class NoCaseLessThan
     {
     public:
         bool operator()(const std::string &lhs, const std::string &rhs) const {