Update for new cppcheck-target-cmake API.
Added UNUSED_CHECK_GROUP to allow the caller to have check groups.

This allows callers to have separated unused function check groups,
useful if the build needs to have completely separate binaries or
libraries which don't interact with each other and need a unique
unused function check.
Added NO_CHECK_UNUSED and NO_CHECK_GENERATED_UNUSED.

These options explicitly prevent all files in a target or at least
the generated files in a target from being added to the unused
function checks.
Merge pull request #29 from smspillaz/common-universal-cmake.item_28

Update for new cppcheck-target-cmake API.