Debugging is enabled with USE_DEBUG not with DEBUG, so change to NUSE_DEBUG.
Index: Makefile
===================================================================
RCS file: /cvs/src/lib/libevent/Makefile,v
retrieving revision 1.30
diff -d -u -p -w -r1.30 Makefile
--- Makefile 21 Apr 2010 21:02:46 -0000 1.30
+++ Makefile 7 Mar 2011 20:31:23 -0000
@@ -57,7 +57,7 @@ MLINKS= event.3 bufferevent_base_set.3 \
event.3 signal_pending.3 \
event.3 signal_set.3
-CFLAGS+= -I${.CURDIR} -DNDEBUG \
+CFLAGS+= -I${.CURDIR} -DNUSE_DEBUG \
-DHAVE_CLOCK_GETTIME \
-DHAVE_CLOCK_MONOTONIC \
-DHAVE_FCNTL_H \
