Check __cpp_lib_string_view, not __cplusplus.

While I'm here, facilitate conversion to std::basic_string_view.

Fixes #361.

Change-Id: I155f7487bdfd9ed65096764a953ac965d8e45f7c
Reviewed-on: https://code-review.googlesource.com/c/re2/+/59770
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed