support a non-hacky way to override link flags

[yihuang]

I have more situations that need to customize the link flags, for example recently I cross compiled the app to mingwW64 with the help of nixpkgs, but there I have to link with -lrocksdb-shared.
currently there's a way to suppress the builtin link flags, which is the testing build tag, it works perfectly right now, it's just the name feels hacky, shall we support sth like grocksdb_no_link build tag, where there's no builtin link flags, user can supply with CGO_LDFLAGS env var?

[linxGnu]

Sound great to me @yihuang please feel free to make a PR

[yihuang]

@linxGnu there's another thing, there are two APIs are not exported on windows(facebook/rocksdb#11217), currently we use a fork to remove those apis to build for windows, do you want me to submit to upstream for windows support?

[linxGnu]

Yes please (although I need to update CI later)