Misc. Cleanups

Reviews: @vinniefalco, @rec
@nbougalis I think I've discovered why I wasn't getting the native CRC32 on my OSX machine when using clang:
https://github.com/facebook/rocksdb/blob/master/util/crc32c.cc#L370
I think I might submit a PR to the RocksDB repo adding clang support for runtime CPUID detection of CRC32 :-)