Summary:
When `PORTABLE=1` is set, RocksDB will now be built with backwards compatibility for MacOS as far back as 10.12 (i.e. 2016).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7016
Reviewed By: ajkr
Differential Revision: D22211312
Pulled By: pdillinger
fbshipit-source-id: 7b0858d9b55d6265d3ea27bf5ea1673639b6538c