Generate a stampfile when checking source-only targets.

This ensures that if the sources don't change, then the target will
not be unconditionally re-run.

Also fixed a bug that caused generated sources to be unconditionally
unchecked. This can now be changed by the user by passing the
CHECK_GENERATED option to the target generators.
Merge pull request #31 from smspillaz/common-universal-cmake.item_30

smspillaz:common-universal-cmake.item_30