Fix missing CoreText symbol in Leopard builds

When building on Leopard a.k.a. Mac OS X 10.5.x, the `kCTForegroundColorFromContextAttributeName` symbol is present in the dylib but not in the supplied headers. This fix should reenable building on a Leopard machine.

Fixes [Graphviz issue 2405](http://www.graphviz.org/mantisbt/view.php?id=2405).