diff --git a/sys/boot/pc98/libpc98/Makefile b/sys/boot/pc98/libpc98/Makefile
index 161bbad311bc..7179559a083c 100644
--- a/sys/boot/pc98/libpc98/Makefile
+++ b/sys/boot/pc98/libpc98/Makefile
@@ -24,6 +24,9 @@ CFLAGS+= -DDISK_DEBUG
 # Include simple terminal emulation (cons25-compatible)
 CFLAGS+= -DTERM_EMU
 
+# XXX: make alloca() useable
+CFLAGS+= -Dalloca=__builtin_alloca
+
 CFLAGS+=	-I${.CURDIR}/../../common -I${.CURDIR}/../btx/lib \
 		-I${.CURDIR}/../../i386/libi386 \
 		-I${.CURDIR}/../../.. -I.