diff --git a/tools/build/Makefile b/tools/build/Makefile
--- a/tools/build/Makefile
+++ b/tools/build/Makefile
@@ -378,7 +378,10 @@
 SYSINCS+=	${SRCTOP}/sys/sys/kbio.h
 # for kldxref:
 SYSINCS+=	${SRCTOP}/sys/sys/module.h
-.if ${.MAKE.OS} != "FreeBSD"
+.if ${.MAKE.OS} == "FreeBSD"
+# for certctl, which relies on modern sysctl.h defining size_t for itself:
+SYSINCS+=	${SRCTOP}/sys/sys/sysctl.h
+.else
 # for libmd:
 SYSINCS+=	${SRCTOP}/sys/sys/md4.h
 .endif