Log Message:
Certain packages dependent upon boost don't automatically do the 'right' thing
with respect to threading, checked in include/boost/config/posix_features.h
so set PTHREAD_AUTO_VARS?=yes directly in buildlink3.mk.
Fixes build in numerous packages such as librevenge and lucene++ as noticed on
SunOS.  No ABI change so no revbump envisioned.