This solves https://github.com/bitcoin-core/leveldb/pull/8
The file env_win.cc is included twice on Windows and once for all other systems. But it is not needed at all there.
This removes this warning on OS X:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ranlib: file: leveldb/libleveldb.a(leveldb_libleveldb_a-env_win.o) has no symbols