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

[bos]

(Imported from Trac #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:          [email protected]
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.