Build: Rework git version detection
Rework the git version detection script to handle some issues that
have been pointed out.
1/ Make it work properly with objdir builds
2/ Don't try to work out if the tree is dirty if git describe failed
3/ Use the configured VERSION as a fallback if we can't obtain proper
version information during a make
Change-Id: I39494e1c18cf4eacbb55386334da7128fbe96310
Reviewed-on: http://gerrit.openafs.org/2283
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>