crosslink html man pages

This is using sed --in-place, which might not be available in
all sed versions. If that's a concern, then please replace with
awk script or move to build-helper and use temporary files.

Also the regex is a heuristic. It works for our man pages here
but it might have false positive matches in the future.

Makefile

@@ -20,6 +20,11 @@
 
 A2XM=a2x -f manpage --no-xmllint -a encoding=UTF-8
 A2XH=a2x -f xhtml --no-xmllint -a encoding=UTF-8
+# Create cross-links in html man pages
+# We look for something like "cdist-type(7)" and make a href out of it
+# The first matching group is the man page name and the second group
+# is the man page section (1 or 7)
+CROSSLINK=sed --in-place 's/\([[:alnum:]_-]*\)(\([17]\))/<a href="..\/man\2\/\1.html">&<\/a>/'
 helper=./bin/build-helper
 
 MANDIR=docs/man
@@ -86,6 +91,7 @@ MANSTATICALL=$(MANSTATICMAN) $(MANSTATICHTML)
 # Creating the type html page
 %.html: %.text
 	$(A2XH) $^
+	$(CROSSLINK) $^
 
 man: $(MANTYPEALL) $(MANREFALL) $(MANSTATICALL)