Remove deprecated boost/utility.

This is enable compatibility with boost 1.69.

simgear/nasal/cppbind/NasalHash.hxx

@@ -24,6 +24,7 @@
 
 #include <simgear/structure/map.hxx>
 #include <boost/iterator/iterator_facade.hpp>
+#include <boost/core/enable_if.hpp>
 
 namespace nasal
 {

simgear/props/props.hxx

@@ -55,13 +55,14 @@ namespace boost {
   struct disable_if : public disable_if_c<Cond::value, T> {};
 }
 #else
-# include <boost/utility.hpp>
 # include <boost/type_traits/is_enum.hpp>
+# include <boost/core/enable_if.hpp>
 
 # include <simgear/debug/logstream.hxx>
 # include <simgear/math/SGMathFwd.hxx>
 # include <simgear/math/sg_types.hxx>
 #endif
+
 #include <simgear/structure/SGReferenced.hxx>
 #include <simgear/structure/SGSharedPtr.hxx>
 

simgear/structure/SGWeakReferenced.hxx

@@ -22,7 +22,7 @@
 #include "SGSharedPtr.hxx"
 
 #include <boost/type_traits/is_base_of.hpp>
-#include <boost/utility/enable_if.hpp>
+#include <boost/core/enable_if.hpp>
 
 #ifdef _MSC_VER
 # pragma warning(push)