debian: Install etc/apt if present (e.g., on Ubuntu)

On Ubuntu, cmake installs are vendor-specific apt.conf.d
snippet, causing the build to fail.

debian/rules

@@ -19,7 +19,8 @@ override_dh_install-indep:
 
 override_dh_install-arch:
 	dh_movefiles
-	dh_install --fail-missing
+	if [ -e debian/tmp/etc/apt ]; then dh_install -papt etc/apt/* etc/apt; fi
+	dh_install --list-missing
 	install -m 644 debian/apt.conf.autoremove debian/apt/etc/apt/apt.conf.d/01autoremove
 	install -m 755 debian/apt.auto-removal.sh debian/apt/etc/kernel/postinst.d/apt-auto-removal