Allow users of _cppcheck_add_checks_to_target to specify when checks run.

For binary targets, it makes sense to set this to PRE_LINK so that failures
will cause a binary build failure when the error bit is set, for custom
targets, it probably makes a lot more sense to set this to PRE_BUILD
as they don't have a linking phase.
Merge pull request #5 from smspillaz/cppcheck-target-cmake.item_5

Allow users of _cppcheck_add_checks_to_target to specify when checks run...