Makefile: Add 'check-rst' target to check .rst files for errors. Also add 'doc-checks' which at the moment just runs check-rst, but might do more in future. fixes #1780

Makefile

@@ -145,6 +145,12 @@ check-umids:
 	-$(PYTHON) misc/coding_tools/check-umids.py `find $(SOURCES) -name '*.py'`
 	@echo
 
+doc-checks: check-rst
+
+check-rst:
+	@for x in `find *.rst docs -name "*.rst"`; do rst2html -v $${x} >/dev/null; done 2>&1 |grep -v 'Duplicate implicit target name:'
+	@echo
+
 count-lines:
 	@echo -n "files: "
 	@find src -name '*.py' |grep -v /build/ |wc -l