quick fix to avoid rebuilding librocksdb.a over and over

A dirty little trick but since we're never gonna be changing _vendor/rocksdb/build_version.cc as much as running tests over and over and over... I think this is well worth our time for now.
We'll still want someone to go over the Makefile at some point, but at least this addresses immediate parts of #38.
We should probably set that environment variable and some others to turn off all the magic in build_detect_platform - it's caused problems before where it will automatically use certain libraries if they're present on the system, even though we don't mean to use them and they make our builds non-reproducible. If we set FBCODE_BUILD=true then it won't auto-detect anything and we can set e.g. the snappy-related variables by hand.