automake: port to Perl 5.22 and later

Without this change, Perl 5.22 complains "Unescaped left brace in
regex is deprecated" and this is planned to become a hard error in
Perl 5.26.  See:
http://search.cpan.org/dist/perl-5.22.0/pod/perldelta.pod#A_literal_%22{%22_should_now_be_escaped_in_a_pattern
* bin/automake.in (substitute_ac_subst_variables): Escape left brace.

bin/automake.in

@@ -3878,7 +3878,7 @@ sub substitute_ac_subst_variables_worker
 sub substitute_ac_subst_variables
 {
   my ($text) = @_;
-  $text =~ s/\${([^ \t=:+{}]+)}/substitute_ac_subst_variables_worker ($1)/ge;
+  $text =~ s/\$[{]([^ \t=:+{}]+)}/substitute_ac_subst_variables_worker ($1)/ge;
   return $text;
 }