Reinterpret Linux paths as MinGW paths under Windows

### 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