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.

.github/workflows/ci.yml

@@ -570,9 +570,6 @@ jobs:
       - name: Config_test nvcc+clang
         run: nvcc -std=c++${{ matrix.std }} --compiler-bindir=clang++ -o config_test -I../../.. config_test.cpp  -latomic && ./config_test
         working-directory: ../boost-root/libs/config/test
-      - name: Config_test nvcc-cuda+clang
-        run: nvcc -c -std=c++${{ matrix.std }} --compiler-bindir=clang++ -I../../.. config_test.cu
-        working-directory: ../boost-root/libs/config/test
       - name: Config_test clang-cuda
         run: clang++ -nocudalib --no-cuda-version-check -c --cuda-gpu-arch=sm_75 -std=c++${{ matrix.std }} -I../../.. config_test.cu
         working-directory: ../boost-root/libs/config/test

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

include/boost/version.hpp

@@ -19,7 +19,7 @@
 //  BOOST_VERSION / 100 % 1000 is the minor version
 //  BOOST_VERSION / 100000 is the major version
 
-#define BOOST_VERSION 108300
+#define BOOST_VERSION 108400
 
 //
 //  BOOST_LIB_VERSION must be defined to be the same as BOOST_VERSION
@@ -27,6 +27,6 @@
 //  number, y is the minor version number, and z is the patch level if not 0.
 //  This is used by <config/auto_link.hpp> to select which library version to link to.
 
-#define BOOST_LIB_VERSION "1_83"
+#define BOOST_LIB_VERSION "1_84"
 
 #endif