Make building with MSVC possible

This makes building with MSVC possible. Tested with MSVC 2019 by doing rebar3 compile, I also tested using this as a dependency in another rebar3 based project (by linking into _checkouts)

One commit is for the erocksdb build scripts and the other one is for adding a flag to rocksdb in order to make it possible to enable find_package for 3rdparty libs (as on other platforms)

Keep in mind that erocksdb tests also need to be fixed since currently a lot of them assume Posix style filesystem layout and put things into /tmp/something.something and perform cleanup using os:cmd("rm -rf /tmp/erocksdb.open.test") which obviously does not work on Windows