diff --git a/lib/lib9p/Makefile b/lib/lib9p/Makefile
index c909bdb1f781..c810ccd79e64 100644
--- a/lib/lib9p/Makefile
+++ b/lib/lib9p/Makefile
@@ -1,10 +1,15 @@
 # $FreeBSD$
 
+.include <src.opts.mk>
+
 .PATH:		${.CURDIR}/../../contrib/lib9p
-CFLAGS+=	-DWITH_CASPER
 CFLAGS+=	-I${.CURDIR}
 CFLAGS+=	-I${.CURDIR}/../../contrib/lib9p
 
+.if ${MK_CASPER} != "no"
+CFLAGS+=	-DWITH_CASPER
+.endif
+
 LIB=		9p
 PACKAGE=	lib${LIB}
 SHLIB_MAJOR=	1