Enable no-fbcode RocksDB build

Summary: I want to use open source build rather than fbcode one. This enables me to run `ROCKSDB_NO_FBCODE=1 make` and run it with my system g++.

Test Plan:
ROCKSDB_NO_FBCODE=1 make
make

Reviewers: sdong, ljin, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23613