build stuff:
 - fix githead_from_path.sh to not use bashisms
 - make it return a slightly longer string so that cmake substring
   for git abbrev will not fail
 - add libntirpc github repo url to error message when submodule not
   here, for people who build without git
 - automatically disable nfsidmap if not found like most other
   features

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>