Remove std::binary_function, which is removed in C++17. Taken from
https://github.com/openbabel/openbabel/pull/2464

Index: src/ops/sort.cpp
--- src/ops/sort.cpp.orig
+++ src/ops/sort.cpp
@@ -29,7 +29,7 @@ namespace OpenBabel
 {
 
 template<class T>
-struct Order : public std::binary_function<std::pair<OBBase*,T>, std::pair<OBBase*,T>, bool>
+struct Order
 {
   Order(OBDescriptor* pDesc, bool rev) : _pDesc(pDesc), _rev(rev){}
   bool operator()(std::pair<OBBase*,T> p1, std::pair<OBBase*,T> p2) const
