Move closer to allowing an "out of the box" distributable build for Mac:

* fix the icon for the .app

* link SDL2 statically, a PR has been sent to the original repo here:
  https://github.com/tcbrindle/sdl2-cmake-scripts/pull/9

* link SFML statically

TODO:

For a releasable build on OS X, system libs must be linked
dynamically while third party libs are linked statically or bundled. The
goal is to link them statically.

The two remaining libs that need static linking are PNG and wX.

For PNG a similar approach to the one used for SDL2 using pkg-config
should work fine and be simple to implement.

For wX things are more complicated. The default build of wX does not
include static libs, I will need to submit a PR for Homebrew to change
the default build to include both dynamic and static versions.