This library is not WARNS=2 clean for -O2 builds, because we include

headers from libpthread that are not WARNS=2 clean for -O2 builds.
Lower the WARNS level to 1. This is the highest level possible for
now.

lib/libthread_db/Makefile

@@ -9,7 +9,7 @@ SRCS+=	libpthread_db.c libpthread_md.c
 SRCS+=	libc_r_db.c libc_r_md.c
 SRCS+=	libthr_db.c
 INCS=	thread_db.h
-WARNS?= 2
+WARNS?= 1
 
 CFLAGS+=-I. -I${.CURDIR}