Makefile: only rebuild definitions when source files change

Makefile

@@ -47,6 +47,7 @@ EEI_FILES:=eei.md
 EEI_SOURCE_FILES:=$(patsubst %, $(eei_submodule)/%, $(EEI_FILES))
 EWASM_FILES:=ewasm-test.md driver.md ewasm.md kewasm-lemmas.md
 EWASM_SOURCE_FILES:=$(EWASM_FILES)
+EXTRA_SOURCE_FILE:=$(patsubst %, $(KWASM_SUBMODULE)/%, $(EEI_SOURCE_FILES) $(EWASM_SOURCE_FILES))
 
 deps:
 	$(KWASM_MAKE) deps
@@ -64,8 +65,7 @@ KOMPILE_OPTS :=
 
 build: build-llvm build-haskell
 
-build-%:
-	cp $(EEI_SOURCE_FILES) $(EWASM_SOURCE_FILES) $(KWASM_SUBMODULE)
+build-%: $(EXTRA_SOURCE_FILES)
 	$(KWASM_MAKE) build-$*                               \
 	    DEFN_DIR=../../$(DEFN_DIR)                       \
 	    llvm_main_module=$(MAIN_MODULE)                  \
@@ -77,6 +77,10 @@ build-%:
 	    EXTRA_SOURCE_FILES="$(EEI_FILES) $(EWASM_FILES)" \
 	    KOMPILE_OPTS="$(KOMPILE_OPTS)"
 
+$(KWASM_SUBMODULE)/%.md: %.md
+	cp $< $@
+
+
 # Testing
 # -------