has_value() is only found in later Boost versions.

Index: src/cpp/core/include/core/http/RequestParser.hpp
--- src/cpp/core/include/core/http/RequestParser.hpp.orig
+++ src/cpp/core/include/core/http/RequestParser.hpp
@@ -66,7 +66,7 @@ class RequestParser (public)
   {
     InputIterator originalBegin = begin;
 
-    if (bufferPos_.has_value())
+    if (bufferPos_)
     {
        if (parsingBody_)
        {
