libthr: increase WARNS to the default (6)

...and silence cast-align warnings from gcc.

Reviewed by:	kib
MFC after:	3 days
Sponsored by:	Dell EMC
Differential Revision:	https://reviews.freebsd.org/D10935

lib/libthr/Makefile

@@ -16,8 +16,8 @@ MK_SSP=	no
 
 LIB=thr
 SHLIB_MAJOR= 3
-WARNS?=	3
 NO_WTHREAD_SAFETY=1
+NO_WCAST_ALIGN.gcc=1    # for gcc 4.2
 CFLAGS+=-DPTHREAD_KERNEL
 CFLAGS+=-I${SRCTOP}/lib/libc/include -I${.CURDIR}/thread \
 	-I${SRCTOP}/include