Patch based on https://git.savannah.gnu.org/cgit/automake.git/commit/?id=13f00eb4493c217269b76614759e452d8302955e

diff -ur automake-1.11.6.orig/automake.in automake-1.11.6/automake.in
--- automake-1.11.6.orig/automake.in	2017-07-28 12:50:12.519289500 -0400
+++ automake-1.11.6/automake.in	2017-07-28 12:56:27.565672500 -0400
@@ -4156,7 +4156,7 @@
 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;
 }