Unfortunately, the autobuilder is using an old version of OSX, and
cannot use ghc newer than the one in lts-13.11. Newer ghc versions use a
libc symbol that is not available.
Since the main stack.yaml has been updated to a newer lts, and just
substituting in lts-13.11 no longer works due to other dependency
issues, it's simplest to use a separate one for the OSX build.
Hopefully this is temporary, and the autobuilder gets updated.
