Fix to be able to build from dir different than source dir

This allows to build different versions of the game (for example, one with RUN_IN_PLACE and one without).

Suggested usage:

<code>mkdir Build
cd Build
cmake .. <options here>
make -j 3
</code>

The fix does not affect people building from the source directory. The executable is placed in a `bin/` subdirectory of the current build dir.