Fix C++ 8.1 build errors. (#207)

Sync of https://github.com/pingcap/rocksdb/pull/22

Fix the following gcc-8 warnings:
- conflicting C language linkage declaration [-Werror]
- writing to an object with no trivial copy-assignment [-Werror=class-memaccess]
- array subscript -1 is below array bounds [-Werror=array-bounds]