Call dh_sphinxdocs only when building arch-indep packages

debian/rules

@@ -7,7 +7,7 @@ DUNE_DEBIAN_DUNECONTROL := $(CURDIR)/bin/dunecontrol
 include debian/dune-debian.mk
 
 %:
-	dh $@ --parallel --builddirectory=build --with sphinxdoc
+	dh $@ --parallel --builddirectory=build
 
 override_dh_auto_build-indep:
 	dh_auto_build -- doc
@@ -17,5 +17,9 @@ override_dh_install-arch:
 	mv debian/tmp/usr/bin/git-whitespace-hook debian/tmp/usr/bin/dune-git-whitespace-hook
 	dh_install -a
 
+override_dh_installdocs-indep:
+	dh_installdocs
+	dh_sphinxdoc
+
 override_dh_strip:
 	dh_strip --dbg-package=libdune-common-dbg