ci: Add `dist-then-build` target to catch missing `EXTRA_DIST`

I personally am very opposed to the entire idea of `make distcheck`;
I think source code should canonically be *git* and not tarballs.
See e.g. https://github.com/cgwalters/git-evtag for some
rationale.

But anyways we are uploading classic tarballs since today that
what Debian/Fedora/etc consume sadly, so we need to test it.

We explicitly skip `make distcheck` since we don't want to rerun
the test suite.

Closes: #1766
Approved by: jlebon

.papr.yml

@@ -47,6 +47,7 @@ tests:
   - ci/ci-commitmessage-submodules.sh
   - ci/build-check.sh
   - ci/ci-release-build.sh
+  - make dist-then-build
 
 artifacts:
   - test-suite.log

Makefile.am

@@ -155,4 +155,14 @@ release-tarball-embedded:
 	mv ostree-embeddeps-$${GITVERSION}.tar{.tmp,}; \
 	gzip -f ostree-embeddeps-$${GITVERSION}.tar 
 
+# `make dist` + `make`; explicitly not the other
+# parts of distcheck like `make uninstall` since
+# we don't care about that.
+dist-then-build: dist
+	rm $(distdir) -rf && tar -xf $(distdir).tar.xz
+	cd $(distdir) && mkdir _build && cd _build && \
+	  ../configure --prefix=/usr --libdir=/usr/lib --sysconfdir=/etc && \
+	  $(MAKE) && make install DESTDIR=$$(pwd)/_install && \
+	rm -rf $(distdir)
+
 -include $(top_srcdir)/git.mk