configure: more helpful "bwidget missing" message
configure: more helpful "libtk-img missing" message
docs: section ids are externally accessible hyperlinks
docs: all html links work now

... So make it a build failure if they stop working in the future.
docs: section ids are externally accessible hyperlinks

(cherry picked from commit 09f7fba10952857ccacd9aea1dc87c186db7f5aa)
docs: all html links work now

... So make it a build failure if they stop working in the future.
(cherry picked from commit bb6cef65dc51386dc0a6b923be0ebb5abea6a873)
Merge remote-tracking branch 'origin/2.6'