Add parallel build files to build usable minimal shared sentencepiece dll with MSVC

[anthonyaue]

The existing Cmake files do not build usable shared libraries with MSVC. These files allow you to build a minimal shared library using MSVC. I tried to just add the relevant changes to the existing CMake files, but was unable to get something that builds everything correctly, so I backed off to checking in these alternatives.

[emjotde]

I am wondering whether we should try to get this into the original repo. The owner is very open to suggestions, see for instance this issue google#252

Although we would probably need to figure this out in a correct way first. Since this does not affect the main build, I am OK with pulling this in, but we should definitely go for a nice build in the original CMakeFile.

We should probably sit down together and take a look where the problems are.

[emjotde]

Approving this so we can move forward, but ultimately, we should have one CMake build that can be shared back with the original repo.

[anthonyaue]

I'd love to get it into the main branch, but we'd need to merge Cmake files and build *.bat files. I'm sure this is doable, but I ran out of patience fighting with CMake+MSVC. The dependency on protobuf makes the whole mess much more complicated because CMake wants to export all of the symbols, including the ones in the C++ runtime library, so you end up with multiple definitions of runtime library symbols while linking. I'll take this as a work item for a later date when I have time to do it right.