Generate enabled_backends in ejabberd.app instead of configure (#3549)

Passing a list of atoms in vars.config.in (introduced in 3c16f214)
breaks rebar2 in "make rel"

configure.ac

@@ -283,17 +283,6 @@ case "`uname`" in
   ;;
 esac
 
-enabled_backends=""
-for backend in odbc mysql pgsql sqlite redis mssql; do
-    if eval test x\${$backend} = xtrue; then
-       if test "x$enabled_backends" = "x"; then
-          enabled_backends=$backend
-       else
-          enabled_backends="$enabled_backends, $backend"
-       fi
-    fi
-done
-
 AC_SUBST(roster_gateway_workaround)
 AC_SUBST(new_sql_schema)
 AC_SUBST(full_xml)
@@ -317,6 +306,5 @@ AC_SUBST(system_deps)
 AC_SUBST(CFLAGS)
 AC_SUBST(CPPFLAGS)
 AC_SUBST(LDFLAGS)
-AC_SUBST(enabled_backends)
 
 AC_OUTPUT

src/ejabberd.app.src.script

@@ -1,8 +1,10 @@
 Vars = case file:consult(filename:join([filename:dirname(SCRIPT), "..", "vars.config"])) of
     {ok, Terms} ->
+        Backends = [mssql, mysql, odbc, pgsql, redis, sqlite],
+        EBs = lists:filter(fun(Backend) -> lists:member({Backend, true}, Terms) end, Backends),
         [lists:keyfind(description, 1, Terms),
          lists:keyfind(vsn, 1, Terms),
-         {env, [lists:keyfind(enabled_backends, 1, Terms)]}
+         {env, [{enabled_backends, EBs}]}
         ];
     _Err ->
         []

vars.config.in

@@ -46,7 +46,6 @@
 
 %% Variables for overlay template files
 {description, "@PACKAGE_NAME@"}.
-{enabled_backends, [@enabled_backends@]}.
 
 %% Platform-specific installation paths
 {release, true}.