debian: Pass -O to make to get readable build logs

Normally make just lets every job write its output directly,
making the log fairly hard to read with high concurrency.

debian/rules

@@ -34,3 +34,6 @@ override_dh_auto_configure-arch: flags=-DWITH_DOC=OFF
 override_dh_auto_configure-indep: flags=-DWITH_DOC=ON
 override_dh_auto_configure-arch override_dh_auto_configure-indep:
 	dh_auto_configure -- $(flags)
+
+override_dh_auto_build:
+	dh_auto_build -- -O