Dpkg::Vendor::Debian: Add new lto feature in new optimize area

This new setting, disabled by default, enables the Link Time
Optimization.

Closes: #940571

man/dpkg-buildflags.pod

@@ -354,6 +354,24 @@ B<LDFLAGS> set to B<-Wl,-z,deb-canary->I<random-id>.
 
 =back
 
+=head2 optimize
+
+Several compile-time options (detailed below) can be used to help optimize
+a resulting binary (since dpkg 1.21.0).
+B<Note>: enabling B<all> these options can result in unreproducible binary
+artifacts.
+
+=over
+
+=item lto
+
+This setting (since dpkg 1.21.0; disabled by default) enables
+Link Time Optimization by adding B<-flto=auto -ffat-lto-objects> to
+B<CFLAGS>, B<CXXFLAGS>, B<OBJCFLAGS>, B<OBJCXXFLAGS>, B<GCJFLAGS>,
+B<FFLAGS>, B<FCFLAGS> and B<LDFLAGS>.
+
+=back
+
 =head2 sanitize
 
 Several compile-time options (detailed below) can be used to help sanitize

scripts/Dpkg/Vendor/Debian.pm

@@ -108,6 +108,9 @@ sub _add_build_flags {
             fixfilepath => 1,
             fixdebugpath => 1,
         },
+        optimize => {
+            lto => 0,
+        },
         sanitize => {
             address => 0,
             thread => 0,
@@ -268,6 +271,13 @@ sub _add_build_flags {
         $flags->append($_, $map) foreach @compile_flags;
     }
 
+    ## Area: optimize
+
+    if ($use_feature{optimize}{lto}) {
+        my $flag = '-flto=auto -ffat-lto-objects';
+        $flags->append($_, $flag) foreach (@compile_flags, 'LDFLAGS');
+    }
+
     ## Area: sanitize
 
     # Handle logical feature interactions.

scripts/t/Dpkg_BuildFlags.t

@@ -16,7 +16,7 @@
 use strict;
 use warnings;
 
-use Test::More tests => 26;
+use Test::More tests => 28;
 
 BEGIN {
     $ENV{DEB_BUILD_ARCH} = 'amd64';
@@ -71,6 +71,9 @@ my %known_features = (
         fixfilepath
         timeless
     ) ],
+    optimize => [ qw(
+        lto
+    ) ],
     sanitize => [ qw(
         address
         leak