Bump minimum boost version for external string_view

src/serialization/string_view.hpp

@@ -21,7 +21,14 @@ that class. */
 
 #include <boost/version.hpp>
 
-#if BOOST_VERSION > 106100
+/*
+ * Boost 1.61 already has string_view, but it fails to compile on some compilers.
+ * The problem is that in string_view::at's ternary expression,
+ * BOOST_THROW_EXCEPTION is not seen as a throw-expression. If a ternary branch
+ * is not a throw-expression, it must be of the same type as the other branch,
+ * necessitating the ', res[0]' workaround.
+ */
+#if BOOST_VERSION > 106400
 
 /* Boost string_view is available, so we can just use it. */
 #include <boost/utility/string_view.hpp>