OpenBSD is not defined (unless sys/param.h is included) so simply use the gcc

builtin define for it.

From:	"Federico G. Schwindt" <fgsch@lodoss.net>

bin/varnishd/mgt/mgt_shmem.c

@@ -247,7 +247,7 @@ mgt_SHM_Create(void)
 		exit (-1);
 	}
 
-#ifdef OpenBSD
+#ifdef __OpenBSD__
 	/* Commit changes, for OS's without coherent VM/buf */
 	AZ(msync(p, getpagesize(), MS_SYNC));
 #endif