autoconf: update pthread checks

autoconf: update pthread checks

Replace obsolete AC_TRY_COMPILE with AC_COMPILE_IFELSE.  Replace shell
if/then conditionals with AS_IF macros.  Reformat indentation and
quoting.

This change was done manually, since autoupdate copes poorly with the
old, nested AC_TRY_COMPILE macros.

Reviewed-on: https://gerrit.openafs.org/13018
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
(cherry picked from commit c5def62d7be4891f534b753374acbf5b524701eb)

Change-Id: I5cc8e5712dd742d20be92d04728eddbecf9c216c
Reviewed-on: https://gerrit.openafs.org/13481
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Marcio Brito Barbosa <mbarbosa@sinenomine.net>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>