Filename case on macOS

In a [project](https://github.com/ivan-m/jbi) I'm working on, I have some directories in the module path consisting solely of capital letters.

When I try and build this project, I get the following warning:
```
:17:10: error:
warning: non-portable path to file '"dist/build/JBI/autogen/cabal_macros.h"'; specified path differs in case from file name on disk [-Wnonportable-include-path]
#include "dist/build/jbi/autogen/cabal_macros.h"
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"dist/build/JBI/autogen/cabal_macros.h"
1 warning generated.
```

(Which I think comes from clang rather than cabal-install itself.)

I also previously had the executable file in `src/jbi.hs`; this resulted in a GHC error:

```
src/JBI.hs:11:8: error:
File name does not match module name:
Saw: ‘Main’
Expected: ‘JBI’
```

I'm not sure if either of these can actually be fixed, or we just have to deal with the joys of caseless filesystems.

cabal-install version 2.0.0.0
compiled using version 2.0.0.2 of the Cabal library

macOS Sierra 10.12.6