Add BOOST_NO_CXX17_INLINE_VARIABLES detection

include/boost/config/compiler/xlcpp.hpp

@@ -23,6 +23,10 @@
 #define __has_extension __has_feature
 #endif
 
+#ifndef __has_cpp_attribute
+#define __has_cpp_attribute(x) 0
+#endif
+
 #if !__has_feature(cxx_exceptions) && !defined(BOOST_NO_EXCEPTIONS)
 #  define BOOST_NO_EXCEPTIONS
 #endif
@@ -242,6 +246,11 @@
 #  define BOOST_NO_CXX17_STRUCTURED_BINDINGS
 #endif
 
+// Clang 3.9+ in c++1z
+#if !__has_cpp_attribute(fallthrough) || __cplusplus < 201406L
+#  define BOOST_NO_CXX17_INLINE_VARIABLES
+#endif
+
 #if !__has_feature(cxx_thread_local)
 #  define BOOST_NO_CXX11_THREAD_LOCAL
 #endif