The resulting Mac wX .app build is now completely independent and
redistributable, only needs to be codesigned.

Necessary dylibs are bundled and linked in a POST_BUILD step using
third_party_libs_tool (included) for which I created a separate repo
here as well:

http://github.com/rkitover/mac-third-party-libs-tool

Turn off Cairo on Mac because it does not work for now.

Set RPATH on the executable to @loader_path/../Frameworks, the bundling
tool also does this.

Update .gitignore for Finder .DS_Store files.

TOOD:

* write a ./quickbuild for Mac and other platforms such as msys2 and
  linux