diff --git a/lib/atf/libatf-c/Makefile b/lib/atf/libatf-c/Makefile
index 2476453749ec..fbdc40497e4b 100644
--- a/lib/atf/libatf-c/Makefile
+++ b/lib/atf/libatf-c/Makefile
@@ -28,9 +28,9 @@
 .include <src.opts.mk>
 .include <bsd.init.mk>
 
-_CFLAGS:=	${CFLAGS}
-_CPPFLAGS:=	${CPPFLAGS}
-_CXXFLAGS:=	${CXXFLAGS}
+ATF_BUILD_CFLAGS:=	${CFLAGS:M-[DILOWf]*}
+ATF_BUILD_CPPFLAGS:=	${CPPFLAGS:M-[DILOWf]*}
+ATF_BUILD_CXXFLAGS:=	${CXXFLAGS:M-[DILOWf]*}
 
 LIB=		atf-c
 PRIVATELIB=	true
@@ -42,11 +42,11 @@ ATF=		${SRCTOP}/contrib/atf
 .PATH:		${ATF}/atf-c/detail
 
 CFLAGS+=	-DATF_BUILD_CC='"${CC}"'
-CFLAGS+=	-DATF_BUILD_CFLAGS='"${_CFLAGS}"'
+CFLAGS+=	-DATF_BUILD_CFLAGS='"${ATF_BUILD_CFLAGS}"'
 CFLAGS+=	-DATF_BUILD_CPP='"${CPP}"'
-CFLAGS+=	-DATF_BUILD_CPPFLAGS='"${_CPPFLAGS}"'
+CFLAGS+=	-DATF_BUILD_CPPFLAGS='"${ATF_BUILD_CPPFLAGS}"'
 CFLAGS+=	-DATF_BUILD_CXX='"${CXX}"'
-CFLAGS+=	-DATF_BUILD_CXXFLAGS='"${_CXXFLAGS}"'
+CFLAGS+=	-DATF_BUILD_CXXFLAGS='"${ATF_BUILD_CXXFLAGS}"'
 CFLAGS+=	-I${ATF}
 CFLAGS+=	-I${.CURDIR}
 CFLAGS+=	-I.