Replace 'if' preprocessor directive with 'ifndef', add OTP_BELOW_24

The 'if' preprocessor directive was added in Erlang/OTP 21.0,
but ejabberd currently supports Erlang 20.0 and higher...
Let's use 'ifndef' instead.

https://www.erlang.org/doc/reference_manual/macros.html#flow-control-in-macros
https://www.erlang.org/doc/apps/compiler/notes#compiler-7.2
https://www.erlang.org/patches/otp-21.0#compiler-7.2

mix.exs

@@ -82,6 +82,7 @@ defmodule Ejabberd.MixProject do
              if_version_below(~c"23", [{:d, :USE_OLD_PG2}]) ++
              if_version_below(~c"24", [{:d, :COMPILER_REPORTS_ONLY_LINES}]) ++
              if_version_below(~c"24", [{:d, :SYSTOOLS_APP_DEF_WITHOUT_OPTIONAL}]) ++
+             if_version_below(~c"24", [{:d, :OTP_BELOW_24}]) ++
              if_version_below(~c"25", [{:d, :OTP_BELOW_25}])
     defines = for {:d, value} <- result, do: {:d, value}
     result ++ [{:d, :ALL_DEFS, defines}]

rebar.config

@@ -129,6 +129,7 @@
             {if_version_below, "23", {d, 'USE_OLD_PG2'}},
             {if_version_below, "24", {d, 'COMPILER_REPORTS_ONLY_LINES'}},
             {if_version_below, "24", {d, 'SYSTOOLS_APP_DEF_WITHOUT_OPTIONAL'}},
+            {if_version_below, "24", {d, 'OTP_BELOW_24'}},
             {if_version_below, "25", {d, 'OTP_BELOW_25'}},
             {if_var_false, debug, no_debug_info},
             {if_var_true, debug, debug_info},

src/mod_matrix_gw.erl

@@ -24,7 +24,7 @@
 %%%----------------------------------------------------------------------
 
 -module(mod_matrix_gw).
--if(?OTP_RELEASE >= 24).
+-ifndef(OTP_BELOW_24).
 
 -author('alexey@process-one.net').
 

src/mod_matrix_gw_room.erl

@@ -24,7 +24,7 @@
 %%%-------------------------------------------------------------------
 -module(mod_matrix_gw_room).
 
--if(?OTP_RELEASE >= 24).
+-ifndef(OTP_BELOW_24).
 -behaviour(gen_statem).
 
 %% API

src/mod_matrix_gw_s2s.erl

@@ -23,7 +23,7 @@
 %%%
 %%%-------------------------------------------------------------------
 -module(mod_matrix_gw_s2s).
--if(?OTP_RELEASE >= 24).
+-ifndef(OTP_BELOW_24).
 -behaviour(gen_statem).
 
 %% API

src/mod_matrix_gw_sup.erl

@@ -20,7 +20,7 @@
 %%%
 %%%----------------------------------------------------------------------
 -module(mod_matrix_gw_sup).
--if(?OTP_RELEASE >= 24).
+-ifndef(OTP_BELOW_24).
 -behaviour(supervisor).
 
 %% API