Dpkg::Vendor::Ubuntu: Fix lto feature to honor DEB_BUILD_OPTIONS

The recently added lto handling for Ubuntu, did not take into account
that the DEB_BUILD_OPTIONS and DEB_BUILD_MAINT_OPTIONS environment
variables are parsed and their options applied just after setting the
defaults, and before applying any arch-specific mask.

We add a new init_build_features() internal method, which sits between
these two actions, so that the Ubuntu vendor module can modify the
defaults before any user or maintainer override are applied.

Fixes: commit df7627ac
Fixes: https://bugs.launchpad.net/bugs/2002582

scripts/Dpkg/Vendor/Debian.pm

@@ -94,6 +94,10 @@ sub run_hook {
     }
 }
 
+sub init_build_features {
+    my ($self, $use_feature, $builtin_feature) = @_;
+}
+
 sub set_build_features {
     my ($self, $flags) = @_;
 
@@ -139,6 +143,8 @@ sub set_build_features {
         },
     );
 
+    $self->init_build_features(\%use_feature, \%builtin_feature);
+
     ## Setup
 
     require Dpkg::BuildOptions;

scripts/Dpkg/Vendor/Ubuntu.pm

@@ -109,17 +109,26 @@ sub run_hook {
 }
 
 # Override Debian default features.
-sub set_build_features {
-    my ($self, $flags) = @_;
+sub init_build_features {
+    my ($self, $use_feature, $builtin_feature) = @_;
 
-    $self->SUPER::set_build_features($flags);
+    $self->SUPER::init_build_features($use_feature, $builtin_feature);
 
     require Dpkg::Arch;
     my $arch = Dpkg::Arch::get_host_arch();
 
     if (any { $_ eq $arch } qw(amd64 arm64 ppc64el s390x)) {
-        $flags->set_feature('optimize', 'lto', 1);
+        $use_feature->{optimize}{lto} = 1;
     }
+}
+
+sub set_build_features {
+    my ($self, $flags) = @_;
+
+    $self->SUPER::set_build_features($flags);
+
+    require Dpkg::Arch;
+    my $arch = Dpkg::Arch::get_host_arch();
 
     if ($arch eq 'ppc64el' && $flags->get_option_value('optimize-level') != 0) {
         $flags->set_option_value('optimize-level', 3);