build: move otp version check to rebar.config.erl

Makefile

@@ -34,7 +34,6 @@ all: $(REBAR) $(PROFILES)
 
 .PHONY: ensure-rebar3
 ensure-rebar3:
-	@$(SCRIPTS)/fail-on-old-otp-version.escript
 	@$(SCRIPTS)/ensure-rebar3.sh $(REBAR_VERSION)
 
 .PHONY: ensure-hex

rebar.config.erl

@@ -3,6 +3,7 @@
 -export([do/2]).
 
 do(Dir, CONFIG) ->
+    ok = assert_otp(),
     case iolist_to_binary(Dir) of
         <<".">> ->
             C1 = deps(CONFIG),
@@ -12,6 +13,22 @@ do(Dir, CONFIG) ->
             CONFIG
     end.
 
+assert_otp() ->
+    Oldest = 23,
+    Latest = 24,
+    OtpRelease = list_to_integer(erlang:system_info(otp_release)),
+    case OtpRelease < Oldest orelse OtpRelease > Latest of
+        true ->
+            io:format(standard_error, "ERROR: Erlang/OTP version ~p found. min=~p, recommended=~p~n",
+                      [OtpRelease, Oldest, Latest]),
+            halt(1);
+        false when OtpRelease =/= Latest ->
+            io:format("WARNING: Erlang/OTP version ~p found, recommended==~p~n",
+                      [OtpRelease, Latest]);
+        false ->
+            ok
+    end.
+
 bcrypt() ->
     {bcrypt, {git, "https://github.com/emqx/erlang-bcrypt.git", {branch, "0.6.0"}}}.
 

scripts/fail-on-old-otp-version.escript

@@ -1,12 +0,0 @@
-#!/usr/bin/env escript
-
-main(_) ->
-    OtpRelease = list_to_integer(erlang:system_info(otp_release)),
-    case OtpRelease < 21 of
-        true ->
-            io:format(standard_error, "ERROR: Erlang/OTP version ~p found. required_min=21, recommended=23~n", [OtpRelease]),
-            halt(1);
-        false ->
-            ok
-    end.
-