Make autopatch of Erlang.mk projects less obtrusive

We do not touch the erlang.mk file anymore. We only patch the
Makefile to include the top-level Erlang.mk by changing the
'include erlang.mk' line to look for ERLANG_MK_FILENAME if it
is defined. The modified Makefile can be committed safely.

core/core.mk

@@ -15,6 +15,7 @@
 .PHONY: all app apps deps search rel relup docs install-docs check tests clean distclean help erlang-mk
 
 ERLANG_MK_FILENAME := $(realpath $(lastword $(MAKEFILE_LIST)))
+export ERLANG_MK_FILENAME
 
 ERLANG_MK_VERSION = rolling
 

core/deps-tools.mk

@@ -74,7 +74,7 @@ endif
 	$(verbose) set -e; for dep in $^ ; do \
 		if ! grep -qs ^$$dep$$ $(ERLANG_MK_RECURSIVE_TMP_LIST); then \
 			echo $$dep >> $(ERLANG_MK_RECURSIVE_TMP_LIST); \
-			if grep -qs -E "^[[:blank:]]*include[[:blank:]]+(erlang\.mk|.*/erlang\.mk)$$" \
+			if grep -qs -E "^[[:blank:]]*include[[:blank:]]+(erlang\.mk|.*/erlang\.mk|.*ERLANG_MK_FILENAME.*)$$" \
 			 $$dep/GNUmakefile $$dep/makefile $$dep/Makefile; then \
 				$(MAKE) -C $$dep fetch-deps \
 				 IS_DEP=1 \

core/deps.mk

@@ -164,11 +164,15 @@ define dep_autopatch_noop
 	printf "noop:\n" > $(DEPS_DIR)/$(1)/Makefile
 endef
 
-# Overwrite erlang.mk with the current file by default.
+# Replace "include erlang.mk" with a line that will load the parent Erlang.mk
+# if given. Do it for all 3 possible Makefile file names.
 ifeq ($(NO_AUTOPATCH_ERLANG_MK),)
 define dep_autopatch_erlang_mk
-	echo "include $(call core_relpath,$(dir $(ERLANG_MK_FILENAME)),$(DEPS_DIR)/app)/erlang.mk" \
-		> $(DEPS_DIR)/$1/erlang.mk
+	$t for f in Makefile makefile GNUmakefile; do \
+		if [ -f $(DEPS_DIR)/$1/$$f ]; then \
+			sed -i.bak s/'include *erlang.mk'/'include $$(if $$(ERLANG_MK_FILENAME),$$(ERLANG_MK_FILENAME),erlang.mk)'/ $(DEPS_DIR)/$1/$$f; \
+		fi \
+	done
 endef
 else
 define dep_autopatch_erlang_mk
@@ -411,7 +415,7 @@ define dep_autopatch_rebar.erl
 			end,
 			[PortSpec(S) || S <- PortSpecs]
 	end,
-	Write("\ninclude $(call core_relpath,$(dir $(ERLANG_MK_FILENAME)),$(DEPS_DIR)/app)/erlang.mk"),
+	Write("\ninclude $$\(if $$\(ERLANG_MK_FILENAME),$$\(ERLANG_MK_FILENAME),erlang.mk)"),
 	RunPlugin = fun(Plugin, Step) ->
 		case erlang:function_exported(Plugin, Step, 2) of
 			false -> ok;

doc/src/guide/deps.asciidoc

@@ -487,10 +487,11 @@ on the kind of project it finds the dependency to be.
 as their build tool. This essentially patches Rebar out, and
 fixes and converts the project to be compatible with Erlang.mk.
 
-* Erlang.mk projects have their 'erlang.mk' file redirect to
-the top-level project's Erlang.mk. This is to ensure that
-functionality works across all dependencies, even if the
-dependency's Erlang.mk is outdated.
+* Erlang.mk projects have their 'Makefile' patched, if necessary,
+to include the top-level project's Erlang.mk. This is to ensure
+that functionality works across all dependencies, even if the
+dependency's Erlang.mk is outdated. The patched Makefile
+can be safely committed if necessary.
 
 * Other Erlang projects get a small Erlang.mk Makefile
 generated automatically.