Add workaround so rebar2 can use Elixir 1.12.0

Notice that Elixir 1.12.0 requires Erlang/OTP 21 or higher
BTW, this workaround works only with rebar2... unfortunately
rebar3 doesn't compile the elixir files from lib/

Makefile.in

@@ -130,6 +130,7 @@ ifeq "$(IS_REBAR)" "3"
   EBINDIR=$(DEPSDIR)/ejabberd/ebin
   REBARREL=$(REBAR) as prod tar
   REBARDEV=REBAR_PROFILE=dev $(REBAR) release
+  ELIXIR_WARNING="\033[32m===>\033[00m If you get \033[1;91mDependency failure: source for elixir...\033[00m, please run \033[4;37mmake\033[00m again"
 else
   SKIPDEPS=skip_deps=true
   LISTDEPS=-q list-deps
@@ -142,12 +143,23 @@ else
   EBINDIR=ebin
   REBARREL=$(REBAR) generate
   REBARDEV=
+  ELIXIR_WARNING="\033[32m===>\033[00m If you get \033[1;91mERROR: Dependency dir .../elixir failed...\033[00m, please run \033[4;37mmake\033[00m again"
 endif
 endif
 
 all: deps src
 
-deps: $(DEPSDIR)/.got
+deps: elixir-rebar $(DEPSDIR)/.got
+
+elixir-rebar:
+	@[ @elixir@ = true ] \
+	 && ( [ ! -d $(DEPSDIR)/elixir/ ] \
+	 && echo $(ELIXIR_WARNING) \
+	      || ( [ ! -f $(DEPSDIR)/elixir/rebar.config ] \
+		   && { mkdir $(DEPSDIR)/elixir/src ; \
+			cp lib/elixir.app.src $(DEPSDIR)/elixir/src/ ; \
+			cp lib/rebar.config $(DEPSDIR)/elixir/ ; } \
+		   || true ) ) || true
 
 $(DEPSDIR)/.got:
 	rm -rf $(DEPSDIR)/.got

lib/elixir.app.src

@@ -0,0 +1,11 @@
+{application, elixir,
+[{description, "elixir"},
+ {vsn, "1.12.0"},
+ {modules, [
+	elixir
+  ]},
+ {registered, [elixir_config, elixir_code_server]},
+ {applications, [kernel,stdlib,compiler]},
+ {mod, {elixir,[]}},
+ {env, [{ansi_enabled, false}]}
+]}.

lib/rebar.config

@@ -0,0 +1,21 @@
+%% Using Elixir as a Rebar dependency
+
+%% This configuration file only exists so Elixir can be used
+%% as a Rebar dependency, the same happens for the file
+%% src/elixir.app.src.
+
+%% In practice, Elixir is structured as OTP where many applications
+%% are placed in the lib directory. Since this structure is not
+%% supported by default by Rebar, after adding Elixir as a dependency
+%% you need to explicitly add it to lib_dirs:
+%%
+%%   {lib_dirs, [
+%%     "deps/elixir/lib"
+%%   ]}.
+%%
+
+%% Run "make" as the proper compilation step
+{post_hooks, [{compile, "make compile"}]}.
+
+%% This prevents rebar_elixir_plugin from recompiling Elixir
+{ex_opts, [{src_dirs, [".PHONY"]}]}.

rebar.config

@@ -24,7 +24,7 @@
         {if_var_true, tools,
          {ejabberd_po, ".*", {git, "https://github.com/processone/ejabberd-po", {branch, "main"}}}},
         {if_var_true, elixir,
-         {elixir, ".*", {git, "https://github.com/elixir-lang/elixir", {tag, "v1.4.4"}}}},
+         {elixir, ".*", {git, "https://github.com/elixir-lang/elixir", {tag, "v1.12.0"}}}},
         {if_var_true, pam,
          {epam, ".*", {git, "https://github.com/processone/epam", {tag, "1.0.10"}}}},
         {if_var_true, redis,