I am uploading a NMU to fix this. The debdiff is attached.
diff -Nru datapacker-1.0.3/debian/changelog
datapacker-1.0.3+nmu1/debian/changelog
--- datapacker-1.0.3/debian/changelog 2021-01-13 07:08:09.000000000 +0100
+++ datapacker-1.0.3+nmu1/debian/changelog 2023-05-19 12:43:26.000000000
+0200
@@ -1,3 +1,11 @@
+datapacker (1.0.3+nmu1) unstable; urgency=medium
+
+ * Non-maintainer upload.
+ * Add missing debian/rules targets build-indep, build-arch. Closes: #999274
+ * Remove unused Build-Depends: gtk-doc-tools, sgml2x.
+
+ -- Bastian Germann <b...@debian.org> Fri, 19 May 2023 12:43:26 +0200
+
datapacker (1.0.3) unstable; urgency=medium
[ Debian Janitor ]
diff -Nru datapacker-1.0.3/debian/control datapacker-1.0.3+nmu1/debian/control
--- datapacker-1.0.3/debian/control 2021-01-13 07:08:09.000000000 +0100
+++ datapacker-1.0.3+nmu1/debian/control 2023-05-19 12:43:26.000000000
+0200
@@ -3,7 +3,6 @@
Priority: optional
Maintainer: John Goerzen <jgoer...@complete.org>
Build-Depends: debhelper-compat (= 12), groff, docbook-utils, man-db,
ghostscript,
- gtk-doc-tools, sgml2x,
ghc (>= 6.8), haskell-devscripts (>= 0.6.6), libghc-missingh-dev (>= 1.0.1),
libghc-unix-dev, libghc-mtl-dev, libghc-filepath-dev, libghc-hslogger-dev
Build-Conflicts: docbook-ebnf
diff -Nru datapacker-1.0.3/debian/rules datapacker-1.0.3+nmu1/debian/rules
--- datapacker-1.0.3/debian/rules 2021-01-13 07:08:09.000000000 +0100
+++ datapacker-1.0.3+nmu1/debian/rules 2023-05-19 12:41:41.000000000 +0200
@@ -89,4 +89,8 @@
dh_builddeb
binary: binary-indep binary-arch
-.PHONY: build clean binary-indep binary-arch binary install configure
+.PHONY: build clean binary-indep binary-arch binary install configure
build-indep build-arch
+
+build-indep:
+
+build-arch: build
