BUILD: makefile: disable threads by default on OpenBSD

As reported by Ilya in issue #725, building with threads on OpenBSD
is broken with gcc:

  include/haproxy/tinfo.h:30: error: thread-local storage not supported for this target

Better stay safe and disable it. Clang seems to support (or emulate)
thread-local, at least it builds. Those willing to experiment can
easily pass USE_THREAD=1.
1 file changed