[PATCH] automake: rewrite scan_variable_expansions regex

The regex pattern in function scan_variable_expansions() fails to
report a portability warning when a dollar-escaped dollar sign
precedes the variable:

	foo_SOURCES = a.c $$$(patsubst a.c,a,b)
---
 lib/Automake/Variable.pm | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib/Automake/Variable.pm b/lib/Automake/Variable.pm
index 4227baad9..4fe1e7284 100644
--- a/lib/Automake/Variable.pm
+++ b/lib/Automake/Variable.pm
@@ -746,9 +746,10 @@ sub scan_variable_expansions ($)
   $text =~ s/#.*$//;
 
   # Record each use of ${stuff} or $(stuff) that does not follow a $.
-  while ($text =~ /(?<!\$)\$(?:\{([^\}]*)\}|\(([^\)]*)\))/g)
+  while ($text =~ m{\$(?:\{([^\}]*)\}|\(([^\)]*)\)|(\$))}g)
     {
-      my $var = $1 || $2;
+      my $var = $1 || $2 || $3;
+      next if $var eq '$';
       # The occurrence may look like $(string1[:subst1=[subst2]]) but
       # we want only 'string1'.
       $var =~ s/:[^:=]*=[^=]*$//;
-- 
2.41.0