dpkg-gencontrol: no longer warns if a substvar provided by -V is not used

The warning is meant to catch unused substitutions coming from the file
and the file is package specific with debhelper.

debian/changelog

@@ -38,6 +38,9 @@ dpkg (1.15.6) UNRELEASED; urgency=low
     compared. Closes: #570008
   * dpkg-gencontrol does no longer accept arch-specific dependencies in
     arch: all packages. Closes: #560071
+  * dpkg-gencontrol no longer warns if a substitution variable provided by -V
+    is not used (the warning is meant to catch unused substitutions coming
+    from the file, those are package specific with debhelper). Closes: #557133
 
   [ Guillem Jover ]
   * Handle argument parsing in dpkg-checkbuilddeps and dpkg-scanpackages

scripts/dpkg-gencontrol.pl

@@ -117,6 +117,7 @@ while (@ARGV) {
         $remove{$1}= 1;
     } elsif (m/^-V(\w[-:0-9A-Za-z]*)[=:]/) {
         $substvars->set($1, $');
+	$substvars->no_warn($1);
     } elsif (m/^-T/) {
         $varlistfile= $';
     } elsif (m/^-n/) {