dpkg-buildpackage: Export all build flags

Export all flags set in Dpkg::BuildFlags to the environment, so that
vendors can usefully define additional variables.

scripts/dpkg-buildpackage.pl

@@ -270,7 +270,7 @@ if (defined $parallel) {
 
 my $build_flags = Dpkg::BuildFlags->new();
 $build_flags->load_config();
-foreach my $flag ("CPPFLAGS", "CFLAGS", "CXXFLAGS", "FFLAGS", "LDFLAGS") {
+foreach my $flag ($build_flags->list()) {
     $ENV{$flag} = $build_flags->get($flag);
     printf(_g("%s: export %s from dpkg-buildflags (origin: %s): %s\n"),
 	   $progname, $flag, $build_flags->get_origin($flag), $ENV{$flag});