Log Message:
Update gtk-doc to 1.21

As per warning in 1.20, gtkdoc-scanobj and generating html via
sgml tools (options.mk) have been removed.

Added

  # needed together with pkg-config by gtkdoc-depscan at runtime
  .include "../../devel/glib2/buildlink3.mk"

to buildlink3.mk

GTK-Doc 1.21    (Jul 17 2014)
============

 Important notice - starting with the next release these long deprecated
 features will be removed, please write to gtk-doc-list@gnome.org and tell us
 your concerns:
 o gtkdoc-mktmpl - please move all the comments into the sources.
 o generating html via sgml tools (jade/openjade), using xsltproc and
   docbook-xslt is pretty common and preferred since version 1.6

 Changes

 o 170860 : gtk-doc should have definitions for stability
 o 644111 : one cannot specify against which libs gtkdoc-fixxref should
            resolve links (problematic with multiple versions)
 o 657444 :  " enum foo { ... } " ; not recognized
 o 671519 : Self-test relies on nonportable (GNU enhanced) 'date' command
 o 678094 : the word "returns" in a function description can be parsed as
            the "Returns:" section
 o 722621 : gtk-doc tarball was created with 32bit uid/gid, unusable with
            mingw/msys tar
 o 724739 : Self-test fail: gtkdoc-mkdb misusing perl datatype
 o 725505 : new syntax highlighting for code is weird
 o 725663 : configure: non POSIX test usage
 o 730658 : Deprecation warning for non-deprecated type GParamFlags
 o 730777 : Add support for nullable and optional annotations