The $(RUSTC_SRC_DL) file already exists, but for some reason Make wants to rebuild
this target when it has $(RUSTC_SRC_TARBALL) as a dependency.

--- a/minicargo.mk
+++ b/minicargo.mk
@@ -220,7 +220,7 @@ $(RUSTC_SRC_DL): rustc-$(RUSTC_VERSION)-src/extracted rustc-$(RUSTC_VERSION)-src
 # - libstd, libpanic_unwind, libtest and libgetopts
 # - libproc_macro (mrustc)
 ifeq ($(USE_MERGED_BUILD),1)
-$(RUSTCSRC)mrustc-stdlib/Cargo.toml: $(RUSTC_SRC_DL) minicargo.mk
+$(RUSTCSRC)mrustc-stdlib/Cargo.toml: minicargo.mk
 	@mkdir -p $(dir $@)
 	@echo "#![no_core]" > $(dir $@)/lib.rs
 	@echo "[package]" > $@