fix for Perl-5.24.0 new deprecation warning

Hi,

on Centos 7.2.1511 with gcc-5.4.0 and Perl-5.24.0 installed,
I ran into a bug in .../automake-1.15/bin/automake, that shows
as failing test in `make check`.
Perl-5.24.0 issues a warning when it encounters a not quoted
left brace ('{') in a regular expression that causes some tests to fail.

Attached is a fix that may be packaged unprofessionally.
The diff was created by having moved the original script to
bin/automake.orig and the fixed version residing in bin/automake.

I think it should be mentioned in the INSTALL file, that configuring and
building automake will give very wrong results, when libtool
is not properly installed on the system.

Best Regards, Armin Faltl

--- bin/automake.orig	2016-10-08 13:34:06.755669580 -0400
+++ bin/automake	2016-10-09 05:08:14.020424529 -0400
@@ -3933,7 +3933,7 @@
 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;
 }