Compilation - at least in MSVC - of custom string_view.hpp was not properly recognising the installed boost version, causing compiler warnings of redefinition of BOOST_CXX14_CONSTEXPR.

src/serialization/string_view.hpp

@@ -19,6 +19,8 @@ that class. */
 
 #pragma once
 
+#include <boost/version.hpp>
+
 #if BOOST_VERSION > 106100
 
 /* Boost string_view is available, so we can just use it. */
@@ -42,7 +44,6 @@ using string_view = boost::string_view;
 #include <cstring>
 #include <iosfwd>
 
-
 #if BOOST_VERSION <= 105600
 # define BOOST_CXX14_CONSTEXPR
 #endif