$OpenBSD: patch-boost_asio_detail_config_hpp,v 1.1 2019/02/06 11:43:35 jca Exp $

libc++-7 deprecates <experimental/string_view> so use <string_view> if present
instead.

libestdc++ provides <string_view> starting with gcc-7.1.0:
  https://github.com/gcc-mirror/gcc/commit/76d7f2c208a23ceeffb5e9b8ebe3ce9cc91c4870#diff-077b1e7070de8a0ee56e6c5c2c4c0cc5

Index: boost/asio/detail/config.hpp
--- boost/asio/detail/config.hpp.orig
+++ boost/asio/detail/config.hpp
@@ -773,7 +773,9 @@
 # if !defined(BOOST_ASIO_DISABLE_STD_STRING_VIEW)
 #  if defined(__clang__)
 #   if (__cplusplus >= 201402)
-#    if __has_include(<experimental/string_view>)
+#    if __has_include(<string_view>)
+#     define BOOST_ASIO_HAS_STD_STRING_VIEW 1
+#    elif __has_include(<experimental/string_view>)
 #     define BOOST_ASIO_HAS_STD_STRING_VIEW 1
 #     define BOOST_ASIO_HAS_STD_EXPERIMENTAL_STRING_VIEW 1
 #    endif // __has_include(<experimental/string_view>)
