Enable cpp(1) warnings in system headers. GCC is oriented on

glibc which is externally maintained, so GCC ships with these
warnings turned off by default.  This is also consistent with
the src/contrib/gcc/c-lex.c,v 1.2 change.

Makefile.inc1

@@ -193,7 +193,7 @@ BMAKEENV=	MAKEOBJDIRPREFIX=${WORLDTMP} \
 BMAKE=		${BMAKEENV} ${MAKE} -f Makefile.inc1 \
 		BOOTSTRAPPING=${OSRELDATE} \
 		-DNOHTML -DNOINFO -DNOMAN -DNOPIC -DNOPROFILE -DNOSHARED \
-		-DNO_CPU_CFLAGS -DNO_WERROR
+		-DNO_CPU_CFLAGS -DNO_WARNS
 
 # build-tool stage
 TMAKEENV=	MAKEOBJDIRPREFIX=${OBJTREE} \

share/mk/bsd.sys.mk

@@ -11,6 +11,7 @@
 .if !defined(NO_WARNS)
 . if defined(WARNS)
 .  if ${WARNS} > 0
+CFLAGS		+=	-Wsystem-headers
 .   if !defined(NO_WERROR)
 CFLAGS		+=	-Werror
 .   endif