docs: Introduce $(devhelphtml_generated)

Our XSLT magic generates one Devhelp-compatible HTML file
per documentation module, but so far we have only shipped
and installed documentation for virterror.

Now that we have $(modules), however, we can generate the
list of files the same way we do for regular documentation
and make sure we always ship and install everything.
Signed-off-by: NAndrea Bolognani <abologna@redhat.com>
Acked-by: NMichal Privoznik <mprivozn@redhat.com>

docs/Makefile.am

@@ -52,7 +52,11 @@ devhelphtml = \
   devhelp/libvirt.devhelp \
   devhelp/index.html \
   devhelp/general.html \
-  devhelp/libvirt-virterror.html
+  $(devhelphtml_generated)
+
+devhelphtml_generated = \
+	$(addprefix devhelp/libvirt-,$(addsuffix .html,$(modules))) \
+	$(NULL)
 
 css = \
   generic.css \