diff -aruN a/bin/automake.in b/bin/automake.in
--- a/bin/automake.in	2013-11-02 17:48:13.000000000 -0700
+++ b/bin/automake.in	2018-09-14 10:21:04.000000000 -0700
@@ -3872,7 +3872,7 @@
 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;
 }