Fix linking problem on OS X for iconv

On OS X Mavericks, the built-in version of iconv is missing the symbol "libiconv_open", resulting in an error like this when you build and test:

```
dyld: lazy symbol binding failed: Symbol not found: _libiconv_open
  Referenced from: /Users/dan/workspace/3dna/rugged/lib/rugged/rugged.bundle
  Expected in: flat namespace

dyld: Symbol not found: _libiconv_open
  Referenced from: /Users/dan/workspace/3dna/rugged/lib/rugged/rugged.bundle
  Expected in: flat namespace

rake aborted!
SIGTRAP
```

The Homebrew version of iconv is compatible, but in order to use that library we need to place system-level frameworks last in the cmake search path. That's what this patch does. I haven't had a chance to test on Linux or other systems so I don't know yet whether it breaks the package there.