Add basic DragonFly support

Saw the build failing in their logs, and it hurt my feelings.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/Makefile b/Makefile
index 55a93fc..8a28f62 100644
--- a/Makefile
+++ b/Makefile
@@ -136,6 +136,10 @@
   LIBS	 += -lpthread -lrt
   LDFLAGS += -rdynamic
 endif
+ifeq ($(CONFIG_TARGET_OS), DragonFly)
+  LIBS	 += -lpthread -lrt
+  LDFLAGS += -rdynamic
+endif
 ifeq ($(CONFIG_TARGET_OS), AIX)
   LIBS	 += -lpthread -ldl -lrt
   CPPFLAGS += -D_LARGE_FILES -D__ppc__