Added BOOST_INLINE_VARIABLE macro.

The BOOST_INLINE_VARIABLE macro can be used to mark C++17 inline variables in
contexts where omitting the inline keyword is still acceptable on older
compilers.

This commit also adds the missing docs for BOOST_NO_CXX17_INLINE_VARIABLES.

include/boost/config/detail/suffix.hpp

@@ -986,6 +986,15 @@ namespace std{ using ::type_info; }
 #define BOOST_CXX14_CONSTEXPR constexpr
 #endif
 
+//
+// C++17 inline variables
+//
+#if !defined(BOOST_NO_CXX17_INLINE_VARIABLES)
+#define BOOST_INLINE_VARIABLE inline
+#else
+#define BOOST_INLINE_VARIABLE
+#endif
+
 //
 // Unused variable/typedef workarounds:
 //