Fix undefined uint128_t type on Windows non-unity builds (#5377)

As part of import optimization, a transitive include had been removed that defined `BOOST_COMP_MSVC` on Windows. In unity builds, this definition was pulled in, but in non-unity builds it was not - causing a compilation error. An inspection of the Boost code revealed that we can just gate the statements by `_MS_VER` instead. A `#pragma message` is added to verify that the statement is only printed on Windows builds.

src/libxrpl/basics/Number.cpp

@@ -31,12 +31,13 @@
 #include <type_traits>
 #include <utility>
 
-#ifdef BOOST_COMP_MSVC
+#ifdef _MSC_VER
+#pragma message("Using boost::multiprecision::uint128_t")
 #include <boost/multiprecision/cpp_int.hpp>
 using uint128_t = boost::multiprecision::uint128_t;
-#else   // !defined(_MSVC_LANG)
+#else   // !defined(_MSC_VER)
 using uint128_t = __uint128_t;
-#endif  // !defined(_MSVC_LANG)
+#endif  // !defined(_MSC_VER)
 
 namespace ripple {