diff --git a/tools/build/Makefile b/tools/build/Makefile
index a701c12a9b5..2c6237c4dd7 100644
--- a/tools/build/Makefile
+++ b/tools/build/Makefile
@@ -351,6 +351,10 @@ MSDOSFSINCS+=	${SRCTOP}/sys/fs/msdosfs/fat.h
 MSDOSFSINCS+=	${SRCTOP}/sys/fs/msdosfs/msdosfsmount.h
 DISKINCS+=	${SRCTOP}/sys/sys/disk/bsd.h
 
+# Needed to build most of the things below, which include sys/cdefs.h either
+# directly or transitively
+SYSINCS+=	${SRCTOP}/sys/sys/cdefs.h
+
 # Needed to build config (since it uses libnv)
 SYSINCS+=	${SRCTOP}/sys/sys/_nv.h
 SYSINCS+=	${SRCTOP}/sys/sys/nv.h ${SRCTOP}/sys/sys/cnv.h \