diff --git a/gnu/lib/libstdc++/Makefile b/gnu/lib/libstdc++/Makefile
index 14bb04191f1f..1005b5d96b54 100644
--- a/gnu/lib/libstdc++/Makefile
+++ b/gnu/lib/libstdc++/Makefile
@@ -22,7 +22,6 @@ CFLAGS+=	-I${GCCLIB}/include -I${SRCDIR}/include -I.
 CFLAGS+=	-frandom-seed=RepeatabilityConsideredGood
 CXXFLAGS+=	-fno-implicit-templates -ffunction-sections -fdata-sections \
 		-Wno-deprecated
-CXXFLAGS:=	${CXXFLAGS:N-stdlib=libc++:N-std=c++[01][13x]:N-std=gnu++[01][13x]}
 PO_CXXFLAGS=	${CXXFLAGS:N-ffunction-sections}
 
 DPADD=		${LIBM}
@@ -626,3 +625,6 @@ ${VERSION_MAP}: ${SRCDIR}/config/abi/pre/gnu.ver
 CLEANFILES+=	${VERSION_MAP}
 
 .include <bsd.lib.mk>
+
+# Filter out libc++-specific flags, and -std= flags above c++98 or gnu++98.
+CXXFLAGS:=	${CXXFLAGS:N-stdlib=libc++:N-std=c++[01][13x]:N-std=gnu++[01][13x]}
diff --git a/gnu/lib/libsupc++/Makefile b/gnu/lib/libsupc++/Makefile
index 76fa1018cab1..90686158bddb 100644
--- a/gnu/lib/libsupc++/Makefile
+++ b/gnu/lib/libsupc++/Makefile
@@ -24,7 +24,6 @@ CFLAGS+=	-I${GCCLIB}/include -I${SRCDIR} -I${GCCDIR}
 CFLAGS+=	-I${.CURDIR}/../libstdc++ -I.
 CFLAGS+=	-frandom-seed=RepeatabilityConsideredGood
 CXXFLAGS+=	-fno-implicit-templates -ffunction-sections -fdata-sections
-CXXFLAGS:=	${CXXFLAGS:N-stdlib=libc++:N-std=c++[01][13x]:N-std=gnu++[01][13x]}
 PO_CXXFLAGS=    ${CXXFLAGS:N-ffunction-sections}
 
 HDRS=	exception new typeinfo cxxabi.h exception_defines.h
@@ -43,3 +42,6 @@ VERSION_MAP=	${.CURDIR}/Version.map
 
 
 .include <bsd.lib.mk>
+
+# Filter out libc++-specific flags, and -std= flags above c++98 or gnu++98.
+CXXFLAGS:=	${CXXFLAGS:N-stdlib=libc++:N-std=c++[01][13x]:N-std=gnu++[01][13x]}