Merge pull request #1375 from natano/configure_bitrig

Add Bitrig to OS detection in configure.ac.

configure.ac

@@ -225,7 +225,7 @@ case "${host_os}" in
             AC_DEFINE(ZMQ_FORCE_MUTEXES, 1, [Force to use mutexes])
         fi
         ;;
-    *openbsd*)
+    *openbsd*|*bitrig*)
         # Define on OpenBSD to enable all library features
         CPPFLAGS="-D_BSD_SOURCE $CPPFLAGS"
         AC_DEFINE(ZMQ_HAVE_OPENBSD, 1, [Have OpenBSD OS])