Add DUK_GIT_DESCRIBE to the public C API
Add DUK_GIT_DESCRIBE to commandline greet line
Release note: DUK_GIT_DESCRIBE
Merge branch 'add-duk-git-describe-macro'

Add DUK_GIT_DESCRIBE macro, useful for snapshot builds so that application
C code can log some git info.  There's no Ecmascript equivalent.