Pass in EXT_DEPS for external dependency tracking.

Makefile.in

@@ -2,17 +2,18 @@ VPATH=%VPATH%
 
 RUSTC ?= rustc
 RUSTFLAGS ?=
+EXT_DEPS ?=
 
 RUST_SRC=$(shell find $(VPATH)/. -type f -name '*.rs')
 
 .PHONY: all
 all:    libio_surface.dummy
 
-libio_surface.dummy: lib.rs $(RUST_SRC)
+libio_surface.dummy: lib.rs $(RUST_SRC) $(EXT_DEPS)
 	$(RUSTC) $(RUSTFLAGS) $< --out-dir .
 	touch $@
 
-io_surface-test: lib.rs $(RUST_SRC)
+io_surface-test: lib.rs $(RUST_SRC) $(EXT_DEPS)
 	$(RUSTC) $(RUSTFLAGS) $< -o $@ --test
 
 .PHONY: check