tk: Default to OS X GUI and fix wish dylib issue

- pass LIB_RUNTIME_DIR to make
- Default to Aqua GUI since X11 is no longer
  shiped with OS X 10.8.
- Using :optional for :x11 so you can install --with-x11

- Fixes:

  dyld: Library not loaded: ...
    Referenced from: /usr/local/bin/wish8.6
    Reason: image not found
  [1]    86386 trace trap  wish8.6