Set WARNS=1.

I believe I've committed all the bits necessary to make this compile
on all supported architectures. :crosses fingers:

lib/msun/Makefile

@@ -36,6 +36,7 @@ ARCH_SUBDIR= ${MACHINE_ARCH}
 LIB=	m
 SHLIBDIR?= /lib
 SHLIB_MAJOR= 5
+WARNS?=	1
 COMMON_SRCS= b_exp.c b_log.c b_tgamma.c \
 	e_acos.c e_acosf.c e_acosh.c e_acoshf.c e_asin.c e_asinf.c \
 	e_atan2.c e_atan2f.c e_atanh.c e_atanhf.c e_cosh.c e_coshf.c e_exp.c \