tags 831953 + patch thanks There is a build target, but there is not a "build-indep" target, so "dh build" seems not to work at all when we create arch-independent packages only (as in this case).
I would suggest removing build target and using override_dh_auto_build instead, as in the attached patch. This is also more "dh"-style. Thanks.
--- a/debian/rules
+++ b/debian/rules
@@ -1,7 +1,7 @@
#!/usr/bin/make -f
-build:
- dh build
+override_dh_auto_build:
+ dh_auto_build
$(MAKE) -C doc
clean:

