-Wsystem-headers, again

[coypu%sdf.org@localhost]

Hi folks.
I wanted to add a C++ test. 
(test addition, still not ATF-ied completely: http://coypu.sdf.org/cabsl.diff)

Unfortunately that resulted in a lot of header warnings from GCC's headers.
http://coypu.sdf.org/cabsl.errlog

The following diff silences the system headers warnings for C++ programs.
OK to commit?

Index: sys.mk
===================================================================
RCS file: /cvsroot/src/share/mk/sys.mk,v
retrieving revision 1.131
diff -u -r1.131 sys.mk
--- sys.mk	20 May 2018 14:39:53 -0000	1.131
+++ sys.mk	19 Jun 2018 18:19:50 -0000
@@ -50,7 +50,7 @@
 CTFMFLAGS	?=	-t -g -L VERSION
 
 CXX?=		c++
-CXXFLAGS?=	${CFLAGS:N-Wno-traditional:N-Wstrict-prototypes:N-Wmissing-prototypes:N-Wno-pointer-sign:N-ffreestanding:N-std=gnu[0-9][0-9]:N-Wold-style-definition:N-Wno-format-zero-length}
+CXXFLAGS?=	${CFLAGS:N-Wno-traditional:N-Wstrict-prototypes:N-Wmissing-prototypes:N-Wno-pointer-sign:N-ffreestanding:N-std=gnu[0-9][0-9]:N-Wold-style-definition:N-Wno-format-zero-length:N-Wsystem-headers}
 
 __ALLSRC1=	${empty(DESTDIR):?${.ALLSRC}:${.ALLSRC:S|^${DESTDIR}|^destdir|}}
 __ALLSRC2=	${empty(MAKEOBJDIR):?${__ALLSRC1}:${__ALLSRC1:S|^${MAKEOBJDIR}|^obj|}}