[Cmake]: Allow utilising simplesquirrel's static library instead of the shared library with a cmake option

[tulpenkiste]

I think this would be handy, especially for linux distributions that won't offer the SuperTux fork of SimpleSquirrel within their repositories.

It could probably be done like this:

cmake_dependent_option(USE_STATIC_SIMPLESQUIRREL "Use static linking for SimpleSquirrel" OFF MSVC ON)