Instead of allowing .depend for specific build-related targets, filter
non-build-related targets (i.e. help, clean) so that other targets can
normally execute .depend target.
closes https://github.com/official-stockfish/Stockfish/pull/4293
No functional change
Add a constraint so that the dependency build only occurs when users
actually run build tasks.
This fixes a bug on some systems where gcc/g++ is not available.
closes https://github.com/official-stockfish/Stockfish/pull/4255
No functional change
