Merge pull request #239 from boostorg/string_view

Add BOOST_NO_CXX17_HDR_STRING_VIEW.

include/boost/config/compiler/visualc.hpp

@@ -203,6 +203,7 @@
 #  define BOOST_NO_CXX17_STRUCTURED_BINDINGS
 #  define BOOST_NO_CXX17_IF_CONSTEXPR
 #  define BOOST_NO_CXX17_HDR_OPTIONAL
+#  define BOOST_NO_CXX17_HDR_STRING_VIEW
 #endif
 
 // MSVC including version 14 has not yet completely

include/boost/config/detail/suffix.hpp

@@ -1026,10 +1026,14 @@ namespace std{ using ::type_info; }
 // This is a catch all case for obsolete compilers / std libs:
 #if !defined(__has_include)
 #  define BOOST_NO_CXX17_HDR_OPTIONAL
+#  define BOOST_NO_CXX17_HDR_STRING_VIEW
 #else
 #if !__has_include(<optional>)
 #  define BOOST_NO_CXX17_HDR_OPTIONAL
 #endif
+#if !__has_include(<string_view>)
+#  define BOOST_NO_CXX17_HDR_STRING_VIEW
+#endif
 #endif
 
 //

include/boost/config/stdlib/libcpp.hpp

@@ -90,6 +90,7 @@
 #if (_LIBCPP_VERSION < 4000) || (__cplusplus <= 201402L)
 #  define BOOST_NO_CXX17_STD_APPLY
 #  define BOOST_NO_CXX17_HDR_OPTIONAL
+#  define BOOST_NO_CXX17_HDR_STRING_VIEW
 #endif
 #if (_LIBCPP_VERSION > 4000) && (__cplusplus > 201402L) && !defined(_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR)
 #  define BOOST_NO_AUTO_PTR

include/boost/config/stdlib/libstdcpp3.hpp

@@ -300,6 +300,7 @@ extern "C" char *gets (char *__s);
 #  define BOOST_NO_CXX17_STD_INVOKE
 #  define BOOST_NO_CXX17_STD_APPLY
 #  define BOOST_NO_CXX17_HDR_OPTIONAL
+#  define BOOST_NO_CXX17_HDR_STRING_VIEW
 #endif
 
 #if defined(__has_include)