When running gtkdocize for a package using GStreamer
I end with a makefile (fragment) with a check like this:


if ENABLE_GTK_DOC
if GTK_DOC_BUILD_HTML
...
endif
if GTK_DOC_BUILD_PDF
...
endif
endif

When running automake I receive error messages
along the lines of:

GTK_DOC_BUILD_HTML does not appear in AM_CONDITIONAL
GTK_DOC_BUILD_PDF does not appear in AM_CONDITIONAL

After an arduous search through autotools-stuff I 
managed to understand that these ought to be set by
gstreamer/common/m4/gtk-doc.m4.

GStreamer 0.11 depends on glib >= 2.13.14 which in
turn depends on gtk-doc 1.18. If I compare
gtk-doc/gtk-doc.m4 and gstreamer/commer/m4/gtk-doc.m4
they differ. Thus my conclusion is that an update of
gtk-doc.m4 (and any related gtk-doc stuff?) in GStreamer
ought to be done. 

My suggested patch updates gstreamer/common/m4/gtk-doc.m4
to that of gtk-doc 1.18. I retain the check for SED as
has previously been added by GStreamer, moreover I add
a short comment explaining why it is needed to ease future
updates of gtk-doc.m4.

I have successfully build gst-plugins-base and gst-plugins-good
using a common with this patch. I hope this is enough to
prove that this patch is benign.