Replace deprecated pre-C++11 code

The functions std::binary_function and std::mem_fun have been deprecated in C++11, and are removed in C++20. This pull request replaces them with more modern code. Together with #1889 this removes all warnings on recent compilers.

common/utils/StringUtils.cpp

@@ -403,12 +403,12 @@ bool HexStringToInt(const string &value, int32_t *output) {
 
 void ToLower(string *s) {
   std::transform(s->begin(), s->end(), s->begin(),
-                 std::ptr_fun<int, int>(std::tolower));
+                 [](int value){return std::tolower(value);});
 }
 
 void ToUpper(string *s) {
   std::transform(s->begin(), s->end(), s->begin(),
-                 std::ptr_fun<int, int>(std::toupper));
+                 [](int value){return std::toupper(value);});
 }
 
 void CapitalizeLabel(string *s) {

include/ola/ExportMap.h

@@ -77,8 +77,7 @@ class BaseVariable {
   std::string m_name;
 };
 
-struct VariableLessThan: public std::binary_function<BaseVariable*,
-                                                     BaseVariable*, bool> {
+struct VariableLessThan {
   bool operator()(BaseVariable *x, BaseVariable *y) {
     return x->Name() < y->Name();
   }

include/olad/Plugin.h

@@ -107,8 +107,7 @@ class AbstractPlugin {
 };
 
 
-struct PluginLessThan: public std::binary_function<AbstractPlugin*,
-                                                   AbstractPlugin*, bool> {
+struct PluginLessThan {
   bool operator()(AbstractPlugin *x, AbstractPlugin *y) {
     return x->Id() < y->Id();
   }