Such a build passes make but make check fails, though.

No, I am not interested in --enable-headless. (I actually think it is a
pointless idea.) I just for fun tried such a build, thinking a bit whether the
LIBO_HEADLESS thing could be replaced by simply !HAVE_FEATURE_DESKTOP,
i.e. setting the latter to zero for a --enable-headless build. But whatever.

Change-Id: I156f956bd2430ba76934774c7d220a1b9f2019bf