Log Message:
Remove build dependency on gtk-doc, related to PR#41475.

It comes from upstream Bug#562310:
http://bugzilla.gnome.org/show_bug.cgi?id=562310
Availability detection of `gtkdoc-rebase' command is depend on the behaviour
of `which' command,  if `which gtkdoc-rebase' return false, do nothing.
It is known as not works as expected on Darwin and Solaris, then failed in
unwanted gtkdoc-rebase execution, and abort.

It is already fixed in upstream gtk-doc repository, but not released yet(1.12?).
So, some packages may contain and be using un-fixed gtk-doc.* files.

For easy to determine whether it is fixed or not, check gtk-doc.make contains
following commit (not using 'which gtkdoc-rebase', but $(GTKDOC_REBASE) macro):

http://git.gnome.org./cgit/gtk-doc/commit/?id=ef5f56ca5fd4a077e6f7a374e8dd333b7a6a8d42