Merge branch 'pthread_barrier' into 'master'

INSTALL.swift

configure.ac

examples/main.c

m4/ax_cc_maxopt.m4

m4/ax_gcc_archflag.m4

m4/ax_pthread.m4

src/Makefile.am

src/barrier.h

src/cell.c

src/common_io.c

src/dump.c

src/dump.h

src/engine.c

src/engine.h

src/logger.c

src/logger.h

src/swift.h

src/threadpool.c

src/threadpool.h

src/version.c