ViSQOL not compiling on mac (and workaround!)

[shenberg]

Recent combinations of macOS/clang fail at building old versions of zlib (<1.3.1) - example github issue.

Workaround:

$ TF_SYSTEM_LIBS=zlib bazel build :visqol -c opt

I had to first expunge my failed build:

$ bazel clean --expunge

Also, bazel on hombrew is too new and old versions there only go back to the 8.x line, I downloaded the version bazel suggested in the error message