deb: make --output-sync=target

Rather than Debian logs containing a barely decipherable mix
of build command and the output of some other command, we
use the make option --output-sync=target. This make the compile
line and the output stay together in the output stream.

This option exists even in the make version in debian;stretch
so should work everywhere.

Test on debian:stretch, ubuntu:18.04, the two lowest version that
use the debian/rules.

debian/rules

@@ -108,7 +108,7 @@ override_dh_auto_build:
 	@echo "RULES.$@"
 	# Print build env info to help debug builds on different platforms
 	dpkg-architecture
-	cd $(BUILDDIR) && $(MAKE)
+	cd $(BUILDDIR) && $(MAKE) --output-sync=target
 
 override_dh_auto_test:
 	@echo "RULES.$@"