You do, indirectly, marshal.h includes it. It dumps an enormous amount of identifiers in the global namespace. The macros are especially awkward, lots of them match names used in the framework.
Lots of things that marshal.h does can be done by the Marshal class as well. But I can’t help you with that, you didn’t mention why you want to use it. You can solve this particular mishap by putting the #include directive before the using statements:
#include <msclr/marshal.h>
using namespace System;
using namespace System::Configuration;