dpkg-buildpackage: Refactor build target fallback code

Move this aside to not clutter the main code flow, to make it easier
to add additional heuristics and to make clear what needs to be
removed once the time comes.

scripts/dpkg-buildpackage.pl

@@ -525,23 +525,9 @@ if (build_has_any(BUILD_SOURCE)) {
 
 run_hook('build', build_has_any(BUILD_BINARY));
 
-if ($buildtarget ne 'build' and scalar(@debian_rules) == 1) {
-    # Verify that build-{arch,indep} are supported. If not, fallback to build.
-    # This is a temporary measure to not break too many packages on a flag day.
-    my $pid = spawn(exec => [ 'make', '-f', @debian_rules, '-qn', $buildtarget ],
-                    from_file => '/dev/null', to_file => '/dev/null',
-                    error_to_file => '/dev/null');
-    my $cmdline = "make -f @debian_rules -qn $buildtarget";
-    wait_child($pid, nocheck => 1, cmdline => $cmdline);
-    my $exitcode = WEXITSTATUS($?);
-    subprocerr($cmdline) unless WIFEXITED($?);
-    if ($exitcode == 2) {
-        warning(g_("%s must be updated to support the 'build-arch' and " .
-                   "'build-indep' targets (at least '%s' seems to be " .
-                   'missing)'), "@debian_rules", $buildtarget);
-        $buildtarget = 'build';
-    }
-}
+# XXX Use some heuristics to decide whether to use build-{arch,indep} targets.
+# This is a temporary measure to not break too many packages on a flag day.
+build_target_fallback();
 
 if (build_has_any(BUILD_BINARY)) {
     withecho(@debian_rules, $buildtarget);
@@ -731,3 +717,24 @@ sub describe_build {
         return g_('full upload (original source is included)');
     }
 }
+
+sub build_target_fallback {
+    return if $buildtarget eq 'build';
+    return if scalar @debian_rules != 1;
+
+    # Check if the build-{arch,indep} targets are supported. If not, fallback
+    # to build.
+    my $pid = spawn(exec => [ 'make', '-f', @debian_rules, '-qn', $buildtarget ],
+                    from_file => '/dev/null', to_file => '/dev/null',
+                    error_to_file => '/dev/null');
+    my $cmdline = "make -f @debian_rules -qn $buildtarget";
+    wait_child($pid, nocheck => 1, cmdline => $cmdline);
+    my $exitcode = WEXITSTATUS($?);
+    subprocerr($cmdline) unless WIFEXITED($?);
+    if ($exitcode == 2) {
+        warning(g_("%s must be updated to support the 'build-arch' and " .
+                   "'build-indep' targets (at least '%s' seems to be " .
+                   'missing)'), "@debian_rules", $buildtarget);
+        $buildtarget = 'build';
+    }
+}