dpkg-buildpackage: Use arch:all in .changes filename when not building arch:any

Closes: #826161

debian/changelog

@@ -39,6 +39,9 @@ dpkg (1.18.8) UNRELEASED; urgency=medium
     Closes: #759999
   * Do not use the debian/rules build target fallback when building both
     architecture independent and dependent packages in dpkg-buipdpackage.
+  * Use architecture «all» as part of the .changes filename when building
+    architecture independent binaries and no architecture dependent binaries.
+    Closes: #826161
   * Perl modules:
     - Use warnings::warnif() instead of carp() for deprecated warnings.
     - Add new format_range() method and deprecate dpkg() and rfc822() methods

scripts/dpkg-buildpackage.pl

@@ -434,12 +434,12 @@ while (<$arch_env>) {
 close $arch_env or subprocerr('dpkg-architecture');
 
 my $arch;
-if (build_is(BUILD_SOURCE)) {
-    $arch = 'source';
-} elsif (build_is(BUILD_ARCH_INDEP)) {
-    $arch = 'all';
-} else {
+if (build_has_any(BUILD_ARCH_DEP)) {
     $arch = mustsetvar($ENV{DEB_HOST_ARCH}, g_('host architecture'));
+} elsif (build_has_any(BUILD_ARCH_INDEP)) {
+    $arch = 'all';
+} elsif (build_has_any(BUILD_SOURCE)) {
+    $arch = 'source';
 }
 
 my $pv = "${pkg}_$sversion";