compilation failure with clang 8 targeting OSX

[BrannonKing]

When cross compiling my software for OSX via Clang 8 I get this error:

In file included from ./hdr/sqlite_modern_cpp/type_wrapper.h:31:
/usr/bin/../include/c++/v1/experimental/optional:18:3: warning: "<experimental/optional> has been removed. Use <optional> instead." [-W#warnings]
# warning "<experimental/optional> has been removed. Use <optional> instead."
  ^
1 warning generated.

In file included from ./hdr/sqlite_modern_cpp.h:14:
./hdr/sqlite_modern_cpp/type_wrapper.h:315:27: error: no member named 'experimental' in namespace 'std'
    using optional = std::experimental::optional<T>;