diff --git a/Makefile.in b/Makefile.in
index b5c9d54..e744ef7 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -2,13 +2,14 @@ VPATH=%VPATH%
 
 RUSTC ?= rustc
 RUSTFLAGS ?=
+EXT_DEPS ?=
 
 RUST_SRC=$(shell find $(VPATH)/. -type f -name '*.rs')
 
 .PHONY: all
 all:    librust-layers.dummy
 
-librust-layers.dummy: lib.rs $(RUST_SRC)
+librust-layers.dummy: lib.rs $(RUST_SRC) $(EXT_DEPS)
 	$(RUSTC) $(RUSTFLAGS) $< --out-dir .
 	touch $@