Fix in-place build by not replacing files with broken symlinks

This fixes the "Please help! I get "/.../colobot/data/levels/po4a-defi101.cfg:3: master file defi101/scene.txt does not exists."

This was due to replacing source files by symlinks in in-source builds.

People should really be doing out-of-source builds, but we should keep in-source builds possible.