automake: port to perl 5.17

[Ingo Brückl]

Unescaped left brace in regex is deprecated and causes a warning.
It will become an error in a later perl release.

Ingo

--- a/bin/automake.in	2015-01-05 20:25:55.000000000 +0100
+++ b/bin/automake.in	2016-02-10 16:37:47.087974806 +0100
@@ -3878,7 +3878,7 @@
 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;
 }