diff --git a/libexec/dma/Makefile.inc b/libexec/dma/Makefile.inc
index e9a0d0a1c614..5d8cfc2c3e19 100644
--- a/libexec/dma/Makefile.inc
+++ b/libexec/dma/Makefile.inc
@@ -4,7 +4,7 @@
 DMA_SOURCES=	${.CURDIR}/../../../contrib/dma
 .PATH:	${DMA_SOURCES}
 
-CFLAGS=	-I${DMA_SOURCES} \
+CFLAGS+= -I${DMA_SOURCES} \
 	-DHAVE_REALLOCF -DHAVE_STRLCPY -DHAVE_GETPROGNAME \
 	-DCONF_PATH='"/etc/dma"' \
 	-DLIBEXEC_PATH='"/usr/libexec"' -DDMA_VERSION='"v0.10"' \