Index: share/mk/src.sys.obj.mk
===================================================================
--- share/mk/src.sys.obj.mk
+++ share/mk/src.sys.obj.mk
@@ -86,6 +86,58 @@
 OBJROOT:=	${OBJTOP}/
 .endif
 
+# Check how to handle default MK_AUTO_OBJ by creating/testing write to all
+# directories needed, and then cleaning up anything we did on exit.
+.if ${MK_AUTO_OBJ} == "no"
+.if !defined(OBJDIR_WRITE_${OBJROOT:hash})
+CheckAutoObj= \
+CheckAutoObj() { \
+	if [ -w "$${1}" ]; then \
+		echo yes; \
+		exit 0; \
+	fi; \
+	trap 'CleanupAutoObj' EXIT; \
+	IFS=/; \
+	d="$${1}"; \
+	set -- $${d}; \
+	for d in "$${@}"; do \
+		dir="$${dir}$${d}/"; \
+		[ -d "$${dir}" ] && continue; \
+		if ! mkdir "$${dir}"; then \
+			echo no; \
+			exit 0; \
+		fi; \
+		created="$${dir} $${created}"; \
+	done; \
+	if [ -w "$${dir}" ]; then \
+		echo yes; \
+		exit 0; \
+	fi; \
+	echo no; \
+	exit 0; \
+}; \
+CleanupAutoObj() { \
+	IFS=" "; \
+	for dir in $${created}; do \
+		rmdir $${dir}; \
+	done; \
+}
+
+OBJDIR_WRITE_${OBJROOT:hash}!= \
+	${CheckAutoObj}; CheckAutoObj "${OBJROOT}" 2>/dev/null || :
+.export OBJDIR_WRITE_${OBJROOT:hash}
+.endif
+.if ${OBJDIR_WRITE_${OBJROOT:hash}} == "yes"
+# Make AUTO_OBJ default and reevaluate it
+__DEFAULT_YES_OPTIONS+=	AUTO_OBJ
+.undef MK_AUTO_OBJ
+.include <bsd.mkopt.mk>
+.if ${MK_AUTO_OBJ} == "yes"
+.export MK_AUTO_OBJ
+.endif
+.endif
+.endif	# !defined(WITHOUT_AUTO_OBJ)
+
 # Assign this directory as .OBJDIR if possible
 .if ${MK_AUTO_OBJ} == "no"
 # The expected OBJDIR already exists, set it as .OBJDIR.