On OS X, warn about filenames that only differ in case

(Imported from [Trac #793](http://hackage.haskell.org/trac/hackage/ticket/793), reported by @tibbe on 2011-01-18)

I managed to make Cabal fail to build a project by having

Name:                hamt

Version:             0.1

License:             BSD3

License-file:        LICENSE

Author:              Johan Tibell

Maintainer:          johan.tibell@gmail.com

Category:            Data

Build-type:          Simple

Cabal-version:       >=1.2

Library

  Exposed-modules: HAMT

  Other-modules: PopCount

  Build-depends: base, vector

  C-sources: popcount.c

The result was:

Building hamt-0.1...

[1 of 2] Compiling PopCount         ( PopCount.hs, dist/build/PopCount.o )

ld: duplicate symbol _popcount in dist/build/popcount.o and dist/build/PopCount.o for inferred architecture i386

_popcount is only defined in popcount.c. Looking in dist/ showed that there was only a popcount.o there (from the C file). Renaming popcount.c to popc.c solved the problem.

Cabal ought to warn in this case.