Thank you for this library!

When using it with mingw, preferred_separator does not get defined:

undefined reference to `ghc::filesystem::path::preferred_separator'

https://ci.appveyor.com/project/sass/libsass/builds/26929504/job/jl88r6p7ci6bq9ad

This is with filesystem.hpp from master, mingw x86_64-4.9.2-release-win32-seh-rt_v4-rev3