Replace std::ignore with boost::ignore_unused()

According to https://isocpp.org/blog/2016/09/quick-q-stdignore-for-ignoring-unused-variable std::ignore is not supposed to be used for this

src/App/FeatureTest.cpp

@@ -22,6 +22,9 @@
 
 
 #include "PreCompiled.h"
+#ifndef _PreComp_
+#include <boost/core/ignore_unused.hpp>
+#endif
 
 #include <Base/Exception.h>
 #include <Base/Unit.h>
@@ -140,10 +143,10 @@ DocumentObjectExecReturn *FeatureTest::execute()
 
     Enumeration enumObj4("Single item");
     enumObj4.setEnums(enums);
-    std::ignore = enumObj4 == enumObj2;
+    boost::ignore_unused(enumObj4 == enumObj2);
     enumObj4.setEnums(nullptr);
     enumObj4 = enumObj2;
-    std::ignore = enumObj4 == enumObj4.getCStr();
+    boost::ignore_unused(enumObj4 == enumObj4.getCStr());
 
     Enumeration enumObj5(enums, enums[3]);
     enumObj5.isValue(enums[2]);

src/App/Metadata.cpp

@@ -25,6 +25,7 @@
 #ifndef _PreComp_
 # include <memory>
 # include <sstream>
+# include <boost/core/ignore_unused.hpp>
 #endif
 
 #include "Metadata.h"
@@ -61,7 +62,7 @@ namespace MetadataInternal {
         void warning(const SAXParseException& toCatch)
         {
             // Don't deal with warnings at all
-            std::ignore = toCatch;
+            boost::ignore_unused(toCatch);
         }
 
         void error(const SAXParseException& toCatch)

src/App/PreCompiled.h

@@ -88,6 +88,7 @@
 // Boost
 #include <boost_signals2.hpp>
 #include <boost/bind/bind.hpp>
+#include <boost/core/ignore_unused.hpp>
 #include <boost/regex.hpp>
 
 #include <boost/utility.hpp>