Add link checker builder, written for GHOP by Thomas Lamb.

Doc/Makefile

@@ -21,6 +21,7 @@ help:
 	@echo "  htmlhelp  to make HTML files and a HTML help project"
 	@echo "  latex     to make LaTeX files, you can set PAPER=a4 or PAPER=letter"
 	@echo "  changes   to make an overview over all changed/added/deprecated items"
+	@echo "  linkcheck to check all external links for integrity"
 
 checkout:
 	@if [ ! -d tools/sphinx ]; then \
@@ -71,6 +72,11 @@ changes: BUILDER = changes
 changes: build
 	@echo "The overview file is in build/changes."
 
+linkcheck: BUILDER = linkcheck
+linkcheck: build
+	@echo "Link check complete; look for any errors in the above or in" \
+	      "build/$(BUILDER)/output.txt"
+
 clean:
 	-rm -rf build/*
 	-rm -rf tools/sphinx

Doc/README.txt

@@ -51,6 +51,10 @@ Available make targets are:
  * "latex", which builds LaTeX source files that can be run with "pdflatex"
    to produce PDF documents.
  
+ * "linkcheck", which checks all external references to see whether they are
+   broken, redirected or malformed, and outputs this information to stdout
+   as well as a plain-text (.txt) file.
+
  * "changes", which builds an overview over all versionadded/versionchanged/
    deprecated items in the current version. This is meant as a help for the
    writer of the "What's New" document.