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.
build_tools/build_detect_platforms invokes build_tools/build_detect_version which rewrites util/build_version.cc on every make invocation. Proper fix would be to make build_tools/build_detect_version slightly more intelligent and have it write to a temporary file and only change util/build_version.cc if the contents have changed. Or you could set the env variable DO_NOT_RUN_BUILD_DETECT_VERSION to non-empty which would cause build_detect_platform to skip running build_detect_version.