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

Index: include/openbabel/plugin.h
--- include/openbabel/plugin.h.orig
+++ include/openbabel/plugin.h
@@ -38,7 +38,7 @@ namespace OpenBabel
 ///@{
 
 /// @brief Case insensitive string comparison for PluginMapType key.
-struct OBERROR CharPtrLess : public std::binary_function<const char*,const char*, bool>
+struct OBERROR CharPtrLess
 {
   bool operator()(const char* p1,const char* p2) const
   { return strcasecmp(p1,p2)<0; }
