Fix jobserver unavailable warnings

When the $(MAKE) is inside an $(eval $(call ...)) then it needs
to be escaped as $$(MAKE).

When the $(MAKE) is inside a $(call ...) then I did not figure
out a way other than passing it as an argument.

When the $(MAKE) is inside many levels of $(call $(call ...))
it's easier to avoid it if at all possible, so I replaced the
rebar $(MAKE) call with ./bootstrap. I confirmed it works fine
on Windows as well.

core/deps.mk

@@ -215,7 +215,7 @@ define dep_autopatch_fetch_rebar2
 		git clone -q -n -- https://github.com/rebar/rebar $(ERLANG_MK_TMP)/rebar; \
 		cd $(ERLANG_MK_TMP)/rebar; \
 		git checkout -q 576e12171ab8d69b048b827b92aa65d067deea01; \
-		$(MAKE); \
+		./bootstrap; \
 		cd -; \
 	fi
 endef

plugins/ct.mk

@@ -54,7 +54,7 @@ endif
 ifneq ($(ALL_APPS_DIRS),)
 define ct_app_target
 apps-ct-$1: test-build
-	$(MAKE) -C $1 ct IS_APP=1
+	$$(MAKE) -C $1 ct IS_APP=1
 endef
 
 $(foreach app,$(ALL_APPS_DIRS),$(eval $(call ct_app_target,$(app))))

test/core_deps.mk

@@ -623,7 +623,7 @@ define add_dep_and_subdep
 	$i "Bootstrap a new OTP library named $(APP)_$(1)subdep"
 	$t mkdir $(APP)_$(1)subdep/
 	$t cp ../erlang.mk $(APP)_$(1)subdep/
-	$t $(MAKE) -C $(APP)_$(1)subdep --no-print-directory -f erlang.mk bootstrap-lib $$v
+	$t $2 -C $(APP)_$(1)subdep --no-print-directory -f erlang.mk bootstrap-lib $$v
 
 	$i "Create a Git repository for $(APP)_$(1)subdep"
 	$t (cd $(APP)_$(1)subdep && \
@@ -636,7 +636,7 @@ define add_dep_and_subdep
 	$i "Bootstrap a new OTP library named $(APP)_$(1)dep"
 	$t mkdir $(APP)_$(1)dep/
 	$t cp ../erlang.mk $(APP)_$(1)dep/
-	$t $(MAKE) -C $(APP)_$(1)dep --no-print-directory -f erlang.mk bootstrap-lib $$v
+	$t $2 -C $(APP)_$(1)dep --no-print-directory -f erlang.mk bootstrap-lib $$v
 
 	$i "Add $(APP)_$(1)subdep as a dependency"
 	$t perl -ni.bak -e \
@@ -655,11 +655,13 @@ endef
 
 core-deps-list-deps: build clean
 
-	$(call add_dep_and_subdep,)
-	$(call add_dep_and_subdep,doc)
-	$(call add_dep_and_subdep,rel)
-	$(call add_dep_and_subdep,test)
-	$(call add_dep_and_subdep,shell)
+# We pass $(MAKE) directly so that GNU Make can pass its context forward.
+# If we didn't then $(MAKE) would be expanded in the call without context.
+	$(call add_dep_and_subdep,,$(MAKE))
+	$(call add_dep_and_subdep,doc,$(MAKE))
+	$(call add_dep_and_subdep,rel,$(MAKE))
+	$(call add_dep_and_subdep,test,$(MAKE))
+	$(call add_dep_and_subdep,shell,$(MAKE))
 
 	$i "Bootstrap a new OTP library named $(APP)"
 	$t mkdir $(APP)/