Update yelp-xsl to 3.8.1

* Added "center" style hint for block title and desc
* Added style hints for new Getting Started designs
* Fixed incorrect automatic link role for guide links
* Support for Mallard conditional language test tokens
* Added style="continues" for Mallard lists and steps
* Better handling of broken internal Mallard links
* MathML support for Mallard and DocBook
* Audio and video support for DocBook
* Ability to set poster image for video
* TTML subtitles for audio and inline media
* Updated media controls
* Fixed #hash auto-expanding and colorizing
* Translator and publisher credits
* Use quote char on :before for blockquote
* ARIA landmark roles
* Changed marker for expanders
* Switched ui:expanded to non-experimental
* Experimental Mallard UI overlays
* Dropped unused DocBook utility XSLT
* Use itstool "join mode" for translations
* More experimental thumbnail link styles
* Many translation updates
Updated textproc/yelp-xsl to 3.8.1
Update yelp to 3.8.1

* Added test token for classic mode (Matthias Clasen)
* Add keywords to the desktop file (Matthias Clasen)
* Fixed handling of xref links with anchors, #686095 (Tails developers)
* Fixed various memory management bugs, #683100 (Carlos Garcia Campos)
* Added local copy of MathJax for MathML display (Shaun McCance)
* Added support for xdg help system in All Documents (Shaun McCance)
* Fixed if:test="action:install" (Shaun McCance)
* Switched to using 'itstool -j' for XSL domain (Shaun McCance)
* Many translation updates
Updated misc/yelp3 to 3.8.1
Fix PLIST.

Mark debug header file as PLIST.debug.
The configure arg --with-driver= takes 'xlib' OR 'osmesa' not 'xlib,osmesa'.