Makefile: validate XML

Make sure we do this before each build.

.gitlab-ci.yml

 job:
   before_script:
     - apt-get -y update
-    - apt-get -y install make python3-sphinx
+    - apt-get -y install make python3-sphinx libxml2-utils
   tags:
     - librem5
   script:

Makefile

 # Makefile for Sphinx documentation
 #
 
+XML_SOURCES = \
+	examples/doap.xml \
+	$(NULL)
+
 # You can set these variables from the command line.
 SPHINXOPTS    = -W
 SPHINXBUILD   = sphinx-build
@@ -49,7 +53,7 @@ clean:
 	rm -rf $(BUILDDIR)/*
 
 .PHONY: html
-html:
+html: check
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."
@@ -223,3 +227,7 @@ dummy:
 	$(SPHINXBUILD) -b dummy $(ALLSPHINXOPTS) $(BUILDDIR)/dummy
 	@echo
 	@echo "Build finished. Dummy builder generates no files."
+
+.PHONY: check
+check:
+	xmllint --noout $(XML_SOURCES)