dpkg-buildpackage: Disable dependency checks on -S -nc

If we are only doing a source build and request not to clean, then
there's no need to check the build dependencies, as clean is the only
thing in a source-only build that requires them.

debian/changelog

@@ -124,6 +124,7 @@ dpkg (1.18.0) UNRELEASED; urgency=low
   * Document that timestamps are reset for files patched with source format
     “3.0 (quilt)” too.
   * Add dpkg --ctrl-tarfile forwarding command for dpkg-deb.
+  * Disable dependency checks on dpkg-buildpackage -S -nc.
 
   [ Raphaël Hertzog ]
   * Drop myself from Uploaders.

scripts/dpkg-buildpackage.pl

@@ -350,6 +350,8 @@ if (($include & BUILD_BINARY) == BUILD_BINARY) {
 if ($noclean) {
     # -nc without -b/-B/-A/-S/-F implies -b
     $include = BUILD_BINARY if build_is_default;
+    # -nc with -S implies no dependency checks
+    $checkbuilddep = 0 if build_sourceonly;
 }
 
 if ($< == 0) {