[CURRENT] r284404 buildworld failure due to CXXFLAGS+= -std=c++11 in /etc/src.conf

Recent source of CURRENT (r284404) fails to buildworld when option 

CXXFLAGS+=             -std=c++11 in /etc/src.conf

is given in /etc/src.conf. That issue was introduced around after
r282336. Below, you'll se the src.conf I use. I also have the buildworl
d failure with src.conf containing only the line CXXFLAGS as specified
above.

With r284336 and before, this issue wasn't present.

Regards,

Oliver


#
CPUTYPE?=               native
#
CFLAGS+=                -O3 -pipe
# for the kernel
#COPTFLAGS+=            -g -O0
COPTFLAGS+=             -O3 -pipe
#
#CXXFLAGS+=             -std=c++11
#
WITH_CLANG_FULL=        YES
WITH_CLANG_EXTRAS=      YES
WITH_LLDB=              YES
#
WITH_IDEA=              YES
#
#WITH_BSD_GREP=         YES
#
#WITH_OFED=             YES
WITH_NAND=              YES
#WITH_CTF=              YES
#
WITH_SVN=               YES
#
MALLOC_PRODUCTION=      YES
#
WITH_ELFTOOLCHAIN_TOOLS=        YES
#
#WITHOUT_DEBUG_FILES=   YES
#
PORTS_MODULES=
#PORTS_MODULES+=                x11/nvidia-driver
#PORTS_MODULES+=                emulators/virtualbox-ose-kmod
#PORTS_MODULES+=                sysutils/acpi_call