add stack.yaml for OSX autobuilder

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.

standalone/osx/stack.yaml

@@ -0,0 +1,33 @@
+flags:
+  git-annex:
+    production: true
+    assistant: true
+    pairing: true
+    s3: true
+    webdav: true
+    torrentparser: true
+    webapp: true
+    magicmime: false
+    dbus: false
+    debuglocks: false
+    benchmark: false
+    networkbsd: false
+    gitlfs: true
+    httpclientrestricted: true
+packages:
+- '.'
+extra-deps:
+ - IfElse-0.85
+ - aws-0.21.1
+ - bloomfilter-2.0.1.0
+ - filepath-bytestring-1.4.2.1.6
+ - sandi-0.5
+ - tasty-rerun-1.1.17
+ - torrent-10000.1.1
+ - git-lfs-1.1.0
+ - http-client-restricted-0.0.2
+ - http-client-0.5.14
+ - magic-1.1
+explicit-setup-deps:
+  git-annex: true
+resolver: lts-13.11