Build static version of lib with cmake

[maxhora]

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[maxhora]

I signed it!

[googlebot]

CLAs look good, thanks!

[djsutherland]

This is similar to the static-lib part of #38, but adds the static lib as a separate target (snappy_static.lib on Windows, libsnappy.a elsewhere). We're (probably) using this version for now in the conda-forge build.

[pwnall]

We decided to use the approach in c756f7f for consistency with other Google projects. (FWIW, my main source of inspiration is googletest.)

If you have some reason that you absolutely need to build both the static and the shared library at the same time, please comment and let me know.