Summary:
I modified the Makefile so that we can compile rocksdb on OpenBSD.
The instructions for building have been added to INSTALL.md.
The whole compilation process works fine like this on OpenBSD-current
Closes https://github.com/facebook/rocksdb/pull/3617

Differential Revision: D7323754

Pulled By: siying

fbshipit-source-id: 990037d1cc69138d22f85bd77ef4dc8c1ba9edea