How to use pthread on DragonFly

pthread linkage happens with -pthread, -lc_r is dropped by libtool
libtool platform change to treat -pthread as library dependency
patch for Python's makesetup script similiar to libtool
thread-using programs MUST link with -pthread, affects Qt and Python linkage

Basic rule:
include mk/pthread.buildlink3.mk
either use ${PTHREAD_LDFLAGS} and ${PTHREAD_LIBS} or PTHREAD_AUTO_VARS, depending on the package