diff --git a/lib/libc++/Makefile b/lib/libc++/Makefile
index 984e70bebe87..5db093488d24 100644
--- a/lib/libc++/Makefile
+++ b/lib/libc++/Makefile
@@ -63,7 +63,7 @@ cxxrt_${_S}: ${_LIBCXXRTDIR}/${_S} .NOMETA
 .endfor
 
 WARNS=		0
-CFLAGS+=	-I${HDRDIR} -I${_LIBCXXRTDIR} -nostdinc++ -nostdlib -DLIBCXXRT
+CFLAGS+=	-isystem ${HDRDIR} -isystem ${_LIBCXXRTDIR} -nostdinc++ -nostdlib -DLIBCXXRT
 .if empty(CXXFLAGS:M-std=*)
 CXXFLAGS+=	-std=c++11
 .endif
diff --git a/lib/libcxxrt/Makefile b/lib/libcxxrt/Makefile
index 15420a692b9f..f2efe8db541f 100644
--- a/lib/libcxxrt/Makefile
+++ b/lib/libcxxrt/Makefile
@@ -21,7 +21,7 @@ SRCS+=		libelftc_dem_gnu3.c\
 		guard.cc
 
 WARNS=		0
-CFLAGS+=	-I${SRCDIR} -nostdinc++
+CFLAGS+=	-isystem ${SRCDIR} -nostdinc++
 .if empty(CXXFLAGS:M-std=*)
 CXXFLAGS+=	-std=c++11
 .endif