pthread.m4: Add missing 'test' to conditional

pthread.m4: Add missing 'test' to conditional

Commit c5def62d (autoconf: update pthread checks) accidentally omitted
a 'test' in one of the conditionals. This causes an ugly error message
during configure:

    checking for pthread_attr_init in -lpthread... yes
    ./configure[31043]: x-lpthread: not found [No such file or directory]

Replace the missing 'test'.

Reviewed-on: https://gerrit.openafs.org/13342
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 38a094137f067255c586dd5c85f3040d7a7c4486)

Change-Id: Iaa7014c6ae58bd492930dbe4f39180111bacb67e
Reviewed-on: https://gerrit.openafs.org/13595
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>