Followup on r1481264: use already existing
platform independent macro instead of pthread
define.

Backport of r1481265 from trunk resp.
r1481266 from 1.5.x.


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.4.x@1481267 13f79535-47bb-0310-9956-ffa450edef68