Pull Request to use tag date instead of last commit date in STAMPED_VERSION

Currently, the STAMPED_VERSION that is printed in each submodule's Makefile is got from the latest commit date, however, if more than one tag point to the same commit hash, the STAMPED_VERSION will be always the same.

The changes proposed here just get the tag date so every time a new tag is created, the STAMPED_VERSION will change, even if it points to the same commit.