Fixes to Tk on OSX

These fixes make Tk compile using `brew` flawlessly on my machine.

* Use the full URL to access the `tcl-tk` formula; no longer need to tap `homebrew-dupes`

* Use opt paths when linking and including; Fixes the X11 headers problem.  This was due to `tcl-tk` being a "keg-only" formula, e.g. it is not linked into `/usr/local/` because it conflicts with OSX's own `tk` installation.

* Use opt paths when dynamically linking, because since `tcl-tk` is "keg-only", it doesn't get linked into `/usr/local/lib`, so we've gotta search in the proper places for them.