Add git tag / SHA1 as a build string

Every build from commit 20271c6 (Apr 10 2013) is versioned as 1.6.0.
Add a build version after the version number
Digital Speech Decoder 1.6.0 (build:v1.6.0-20-g16110bd)

The build string uses git describe to list the tag closest to head,
and the number of commits past that tag, the a literal g and ending
with the current SHA1

  * Note the v1.6.0 tag was linked to commit 5d51df0, that commit is
detached from head, moved the tag to commit 5d147c9f. Describe would
not list that tag as it is not reachable from head.
 ** Does not look like my pull request includes the tags, so some one with direct access to the repo will need to delete v1.6.0 and then re add it to the correct commit (or tag a new v1.6.1 ). If the tag is not moved git will report last tag as 1.4.0.

  * Two of the helper files cmake/git_revision.cmake and cmake/git_revision.cmake.in are licensed under a Boost Software License ( http://www.boost.org/LICENSE_1_0.txt ), its looks open and free.