Index: bin/hdrtest/Makefile
===================================================================
--- /dev/null
+++ bin/hdrtest/Makefile
@@ -0,0 +1,44 @@
+#
+# A number of header files in sys/* have, going back to 7th Edition Unix in
+# 1979, reqiured other files (like sys/types.h) to compile. Likewise the 4BSD
+# networking code has had prerequisites. However, going back to around the turn
+# of the century, other systems have made them be independently includable
+# (wide-spread header include protection post-dates 7th edition Unix by maybe 4
+# or 5 years judging from netnews sources). Start down the path of making them
+# all independently includable by creating this test that fails buildworld when
+# they are not.
+#
+PROG=	hdrtest
+SRCS=	hdrtest.c
+INTERNALPROG=	This is a compile-only test
+MAN=
+
+# Some files have to be clean for extra defines too...
+#CFLAGS.event.c=	-D_WANT_KEVENT32 -D_WANT_FREEBSD11_KEVENT
+
+.include "goodfiles.inc"
+
+.for h in ${HDRS}
+SRCS+=	${h:R}.c
+CLEANFILES+=${h:R}.c
+${h:R}.c:
+	echo "#include <$h>" > ${.TARGET}
+.endfor
+
+#
+# Target to make the current known good list. Only run it when this hdrtest is
+# building so the list never shrinks (except when files are removed entirely
+# that are on the list).
+#
+goodfiles.inc!
+	@(cd ${SRCTOP}/sys; \
+	echo "# DO NOT EDIT-- this file is automatically @generated."; \
+	echo "HDRS= \\"; \
+	for i in sys/*.h; do \
+		echo "#include <$$i>" | \
+			${CC} ${CFLAGS} -xc - -c -o /dev/null 2> /dev/null && \
+			echo "	$$i \\"; \
+	done; \
+	echo) > ${.CURDIR}/goodfile.inc
+
+.include <bsd.prog.mk>