Make arg_parser global static

The usage of ap_is_used() requires that the arg_parser is globally
accessible
Fix warning while building on OS X
Autoconf - checking for pthread_barrier
Autoconf - ease check for clock_gettime
Update Doxyfile
Fix doxygen
Autoconf - update checking for pthread_barrier
Fix warning while building on OS X
Missing pthread barrier implemenation for OS X
Merge remote-tracking branch 'upstream/next' into next

Conflicts:
	src/flowgrind.c
Merge pull request #141 from alexander-zimmermann/next

Pthread barrier implemenation for OS X