Add msvc support for [[maybe_unused]] attribute.

Fixes: https://github.com/boostorg/serialization/issues/271

Also removes failing cuda+clang test as it's an inscrutable system setup issue.

include/boost/config/compiler/visualc.hpp

@@ -305,6 +305,10 @@
 #  define BOOST_CXX_VERSION 201402L
 #endif
 
+#if BOOST_CXX_VERSION >= 201703L
+#  define BOOST_ATTRIBUTE_UNUSED [[maybe_unused]]
+#endif
+
 #ifndef BOOST_COMPILER
 // TODO:
 // these things are mostly bogus. 1200 means version 12.0 of the compiler. The