Stack: Reinterpret Linux paths as MinGW paths under Windows

[varosi]

General summary/comments

Interpret Linux paths, like:

/opt/OpenBLAS/bin

as

C:\Users\MyUser\AppData\Local\Programs\stack\x86_64-windows\msys2-20150512\opt\OpenBLAS\bin

Steps to reproduce

1. Try to build hmatrix library under Windows

Expected

Linux paths in .yaml files under Windows to be reinterperted.

Stack version

$ stack --version
Version 1.4.0, Git revision e714f1dd3fade19496d91bd6a017e435a96a6bcd x86_64 hpack-0.17.0

Method of installation

• Official binary, downloaded from stackage.org or fpcomplete's package repository

[mgsloan]

I can see something like that making sense for linux-style absolute paths. Seems like a good idea if it works for many cases.