debian: use bootstrap.sh -j

We believe it to be safe now.  In test run on multi-core machines it
is producing bit-for-bit identical result compared to the non-parallel
invocation.

debian/rules

@@ -47,7 +47,7 @@ override_dh_auto_clean:
 
 .stamp-bootstrap:
 	@$(call show_vars)
-	./bootstrap.sh
+	./bootstrap.sh -j
 	touch $@
 
 .stamp-configure: .stamp-bootstrap