(folly osx) test for the zip, not the dir

(folly osx) test for the zip, not the dir

Summary: Apparently `configure` creates this subdirectory and then the curl and unzip commands don't run, and then `make check` fails. Instead, do it if the zip file doesn't exist.

Test Plan:
git clone https://github.com/facebook/folly.git
cd folly/folly
/path/to/this/bootstrap-osx-homebrew.sh
make
make check

Reviewed By: andrei.alexandrescu@fb.com

Subscribers: exa, folly-diffs@, yfeldblum, davejwatson

FB internal diff: D1882472

Signature: t1:1882472:1425920470:bdab25967d3b8e52c690d5627168077c65829b92