Fix ghci dependency on iconv on 10.9.

Compiling GHC against the stock iconv provided by OS X Mavericks causes the following:
```
$ ghci
GHCi, version 7.6.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... <command line>: can't load .so/.DLL for: /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib (dlopen(/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib, 9): no suitable image found.  Did find:
	/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/lib/libiconv.dylib: mach-o, but wrong filetype)
```

This commit solves the problem using the kegg-only version of libiconv.