LU-1199 build: Improve git hook link installation

LU-1199 build: Improve git hook link installation

Check for .git/hooks directory before trying anything else.  Removes
one barrier to compiling lustre without .git being present.

Change-Id: I5fdf8fae9a4958099a40772d927fa59e42446a5b
Signed-off-by: Christopher J. Morrone <morrone2@llnl.gov>
Reviewed-on: http://review.whamcloud.com/5879
Tested-by: Hudson
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>