Linux: Add autoconf macro for structure checks

Linux: Add autoconf macro for structure checks

Add a new autoconf macro for doing structure element checks.

Reviewed-on: http://gerrit.openafs.org/1853
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 02f2c7cb3734d44dc90b77d631909373daefacd3)

Change-Id: I3639293928a821ba41456b79fd1425711f6e36f4
[andersk@mit.edu: Remove modifications of existing structure checks]
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-on: http://gerrit.openafs.org/3992
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Russ Allbery <rra@stanford.edu>
(cherry picked from commit 166019b13bf9e9be3983533ff59ae4a23e4cc9ea)