Summary:
- downcase includes for case-sensitive filesystems
- give targets the same name (librocksdb) on all platforms

With this patch it is possible to cross-compile RocksDB for Windows
from a Linux host using mingw.

cc yuslepukhin orgads
Closes https://github.com/facebook/rocksdb/pull/2107

Differential Revision: D4849784

Pulled By: siying

fbshipit-source-id: ad26ed6b4d393851aa6551e6aa4201faba82ef60