diff --git a/tools/mk/autobuild.mk b/tools/mk/autobuild.mk
new file mode 100644
index 000000000..12890c7cb
--- /dev/null
+++ b/tools/mk/autobuild.mk
@@ -0,0 +1,18 @@
+# Source files located under $(AUTOBUILD_ROOT) are automatically added.
+ifeq ($(AUTOBUILD_ROOT),)
+  AUTOBUILD_ROOT := ./source/
+endif
+
+rwildcard   := $(wildcard $1$2) $(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2))
+AUTOCSRC    := $(call rwildcard,$(AUTOBUILD_ROOT),*.c)
+AUTOCPPSRC  := $(call rwildcard,$(AUTOBUILD_ROOT),*.cpp)
+AUTOASMSRC  := $(call rwildcard,$(AUTOBUILD_ROOT),*.s)
+AUTOXASMSRC := $(call rwildcard,$(AUTOBUILD_ROOT),*.S)
+AUTOINC     := $(sort $(dir $(call rwildcard,$(AUTOBUILD_ROOT),*)))
+
+# Shared variables.
+ALLCSRC    += $(AUTOCSRC)
+ALLCPPSRC  += $(AUTOCPPSRC)
+ALLASMSRC  += $(AUTOASMSRC)
+ALLXASMSRC += $(AUTOXASMSRC)
+ALLINC     += $(AUTOINC)